Add a unit test for invoking saveOverlays() with a null value in the Map

Author: dconeybe

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

@@ -23,6 +23,7 @@
 import static com.google.firebase.firestore.testutil.TestUtil.setMutation;
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 
 import com.google.firebase.firestore.auth.User;
@@ -41,6 +42,7 @@
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
+import org.junit.function.ThrowingRunnable;
 
 /**
  * These are tests for any implementation of the DocumentOverlayCache interface.
@@ -252,6 +254,24 @@ public void testUpdateDocumentOverlay() {
     assertNull(cache.getOverlay(DocumentKey.fromPathString("coll/doc")));
   }
 
+  @Test
+  public void testSaveOverlaysThrowsNullPointerExceptionOnNullMapValue() {
+    Map<DocumentKey, Mutation> data = new HashMap<>();
+    data.put(key("coll/doc"), null);
+
+    NullPointerException e =
+        assertThrows(
+            NullPointerException.class,
+            new ThrowingRunnable() {
+              @Override
+              public void run() {
+                cache.saveOverlays(1, data);
+              }
+            });
+
+    assertThat(e.getMessage()).contains("coll/doc");
+  }
+
   void verifyOverlayContains(Map<DocumentKey, Overlay> overlays, String... keys) {
     Set<DocumentKey> expected = Arrays.stream(keys).map(TestUtil::key).collect(Collectors.toSet());
     assertThat(overlays.keySet()).containsExactlyElementsIn(expected);