Fix bug

Author: schmidt-sebastian

firebase-firestore/src/main/java/com/google/firebase/firestore/model/DocumentKey.java

@@ -51,7 +51,7 @@ public static DocumentKey empty() {
   public static DocumentKey fromName(String name) {
     ResourcePath resourceName = ResourcePath.fromString(name);
     hardAssert(
-        resourceName.length() >= 4
+        resourceName.length() > 4
             && resourceName.getSegment(0).equals("projects")
             && resourceName.getSegment(2).equals("databases")
             && resourceName.getSegment(4).equals("documents"),