Replace Collection.emptyMap() with new HashMap() (firebase#5573)

Author: milaGGL

firebase-firestore/CHANGELOG.md

@@ -1,4 +1,6 @@
 # Unreleased
+* [fixed] Fixed an issue caused by calling mutation on immutable map object. [#5573](//github.com/firebase/firebase-android-sdk/pull/5573)
+
 * [fixed] Fixed an issue in the local cache synchronization logic where all locally-cached documents that matched a resumed query would be unnecessarily re-downloaded; with the fix it now only downloads the documents that are known to be out-of-sync. [#5506](//github.com/firebase/firebase-android-sdk/pull/5506)
 * [fixed] Fixed an issue where GC runs into a infinite loop in a certain case. [#5417](https://github.com/firebase/firebase-android-sdk/issues/5417)
 

firebase-firestore/src/main/java/com/google/firebase/firestore/local/LocalDocumentsView.java

@@ -340,7 +340,7 @@ LocalDocumentsResult getNextDocuments(String collectionGroup, IndexOffset offset
         count - docs.size() > 0
             ? documentOverlayCache.getOverlays(
                 collectionGroup, offset.getLargestBatchId(), count - docs.size())
-            : Collections.emptyMap();
+            : new HashMap<>();
 
     int largestBatchId = FieldIndex.INITIAL_LARGEST_BATCH_ID;
     for (Overlay overlay : overlays.values()) {