Support empty map in Firestore bundle (#12312)

Author: cherylEnkidu

Firestore/core/src/bundle/bundle_serializer.cc

@@ -584,10 +584,16 @@ Message<google_firestore_v1_Value> BundleSerializer::DecodeValue(
 
 Message<google_firestore_v1_MapValue> BundleSerializer::DecodeMapValue(
     JsonReader& reader, const json& map_json) const {
-  if (!map_json.is_object() || !map_json.contains("fields")) {
-    reader.Fail("mapValue is not a valid map");
+  if (!map_json.is_object()) {
+    reader.Fail("mapValue is not a valid object");
     return {};
   }
+
+  // Empty map doesn't have `fields` field.
+  if (!map_json.contains("fields")) {
+    return {};
+  }
+
   const auto& fields = map_json.at("fields");
   if (!fields.is_object()) {
     reader.Fail("mapValue's 'field' is not a valid map");

Firestore/core/test/unit/bundle/bundle_reader_test.cc

@@ -231,10 +231,13 @@ class BundleReaderTest : public ::testing::Test {
     value3.set_null_value(google::protobuf::NULL_VALUE);
     ProtoValue value4;
     value4.mutable_array_value();
+    ProtoValue value5;
+    value5.mutable_map_value();
     document.mutable_fields()->insert({"\0\ud7ff\ue000\uffff\"", value1});
     document.mutable_fields()->insert({"\"(╯°□°）╯︵ ┻━┻\"", value2});
     document.mutable_fields()->insert({"nValue", value3});
     document.mutable_fields()->insert({"emptyArray", value4});
+    document.mutable_fields()->insert({"emptyMap", value5});
 
     return document;
   }