Make TransformResult non-nullable

Gil asked that this should not be part of #2383, but I think it still makes sense as an indepedent simplification

Author: schmidt-sebastian

firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/DeleteMutation.java

@@ -59,7 +59,7 @@ public MaybeDocument applyToRemoteDocument(
     verifyKeyMatches(maybeDoc);
 
     hardAssert(
-        mutationResult.getTransformResults() == null,
+        mutationResult.getTransformResults().isEmpty(),
         "Transform results received by DeleteMutation.");
 
     // Unlike applyToLocalView, if we're applying a mutation to a remote document the server has

firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/MutationResult.java

@@ -31,9 +31,9 @@
  */
 public final class MutationResult {
   private final SnapshotVersion version;
-  @Nullable private final List<Value> transformResults;
+  private final List<Value> transformResults;
 
-  public MutationResult(SnapshotVersion version, @Nullable List<Value> transformResults) {
+  public MutationResult(SnapshotVersion version, List<Value> transformResults) {
     this.version = checkNotNull(version);
     this.transformResults = transformResults;
   }
@@ -58,9 +58,8 @@ public SnapshotVersion getVersion() {
    * The resulting fields returned from the backend after a mutation containing field transforms has
    * been committed. Contains one Value for each FieldTransform that was in the mutation.
    *
-   * <p>Will be null if the mutation did not contain any field transforms.
+   * <p>Returns an empty list if the mutation did not contain any field transforms.
    */
-  @Nullable
   public List<Value> getTransformResults() {
     return transformResults;
   }

firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/PatchMutation.java

@@ -117,11 +117,7 @@ public MaybeDocument applyToRemoteDocument(
       return new UnknownDocument(this.getKey(), mutationResult.getVersion());
     }
 
-    List<Value> transformResults =
-        mutationResult.getTransformResults() != null
-            ? serverTransformResults(maybeDoc, mutationResult.getTransformResults())
-            : new ArrayList<>();
-
+    List<Value> transformResults = serverTransformResults(maybeDoc, mutationResult.getTransformResults());
     SnapshotVersion version = mutationResult.getVersion();
     ObjectValue newData = patchDocument(maybeDoc, transformResults);
     return new Document(getKey(), version, newData, Document.DocumentState.COMMITTED_MUTATIONS);

firebase-firestore/src/main/java/com/google/firebase/firestore/model/mutation/SetMutation.java

@@ -81,12 +81,9 @@ public MaybeDocument applyToRemoteDocument(
 
     SnapshotVersion version = mutationResult.getVersion();
 
-    ObjectValue newData = value;
-    if (mutationResult.getTransformResults() != null) {
-      List<Value> transformResults =
-          serverTransformResults(maybeDoc, mutationResult.getTransformResults());
-      newData = transformObject(newData, transformResults);
-    }
+    List<Value> transformResults =
+            serverTransformResults(maybeDoc, mutationResult.getTransformResults());
+    ObjectValue newData = transformObject(value, transformResults);
 
     return new Document(getKey(), version, newData, Document.DocumentState.COMMITTED_MUTATIONS);
   }

firebase-firestore/src/main/java/com/google/firebase/firestore/remote/RemoteSerializer.java

@@ -445,13 +445,10 @@ public MutationResult decodeMutationResult(
       version = commitVersion;
     }
 
-    List<Value> transformResults = null;
     int transformResultsCount = proto.getTransformResultsCount();
-    if (transformResultsCount > 0) {
-      transformResults = new ArrayList<>(transformResultsCount);
-      for (int i = 0; i < transformResultsCount; i++) {
-        transformResults.add(proto.getTransformResults(i));
-      }
+    List<Value> transformResults = new ArrayList<>(transformResultsCount);
+    for (int i = 0; i < transformResultsCount; i++) {
+      transformResults.add(proto.getTransformResults(i));
     }
     return new MutationResult(version, transformResults);
   }

firebase-firestore/src/test/java/com/google/firebase/firestore/local/LocalStoreTestCase.java

@@ -159,7 +159,7 @@ private void acknowledgeMutation(long documentVersion, @Nullable Object transfor
             version,
             transformResult != null
                 ? Collections.singletonList(TestUtil.wrap(transformResult))
-                : null);
+                : Collections.emptyList());
     MutationBatchResult result =
         MutationBatchResult.create(
             batch, version, singletonList(mutationResult), WriteStream.EMPTY_STREAM_TOKEN);

firebase-firestore/src/test/java/com/google/firebase/firestore/model/MutationTest.java

@@ -597,8 +597,7 @@ public void testTransitions() {
         doc("collection/key", 7, map(), Document.DocumentState.COMMITTED_MUTATIONS);
     UnknownDocument docV7Unknown = unknownDoc("collection/key", 7);
 
-    MutationResult mutationResult = new MutationResult(version(7), /*transformResults=*/ null);
-    MutationResult transformResult = new MutationResult(version(7), Collections.emptyList());
+    MutationResult mutationResult = new MutationResult(version(7), /*transformResults=*/ Collections.emptyList());
 
     assertVersionTransitions(set, docV3, mutationResult, docV7Committed);
     assertVersionTransitions(set, deletedV3, mutationResult, docV7Committed);

firebase-firestore/src/test/java/com/google/firebase/firestore/spec/SpecTestCase.java

@@ -712,7 +712,7 @@ private void doWriteAck(JSONObject writeAckSpec) throws Exception {
     validateNextWriteSent(write.first);
 
     MutationResult mutationResult =
-        new MutationResult(version(version), /*transformResults=*/ null);
+        new MutationResult(version(version), /*transformResults=*/ Collections.emptyList());
     queue.runSync(() -> datastore.ackWrite(version(version), singletonList(mutationResult)));
   }
 

firebase-firestore/src/testUtil/java/com/google/firebase/firestore/testutil/TestUtil.java

@@ -550,7 +550,7 @@ public static VerifyMutation verifyMutation(String path, int micros) {
   }
 
   public static MutationResult mutationResult(long version) {
-    return new MutationResult(version(version), null);
+    return new MutationResult(version(version), Collections.emptyList());
   }
 
   public static LocalViewChanges viewChanges(