More ServerTimestamp fixes

Author: schmidt-sebastian

firestore/src/main/field_value_main.cc

@@ -165,13 +165,18 @@ double FieldValueInternal::double_increment_value() const {
 // Creating sentinels
 
 FieldValue FieldValueInternal::Delete() {
-  return MakePublic(
-      FieldValueInternal{Type::kDelete, Message<google_firestore_v1_Value>{}});
+  Message<google_firestore_v1_Value> value;
+  value->which_value_type = google_firestore_v1_Value_integer_value_tag;
+  value->map_value = {};
+  return MakePublic(FieldValueInternal{Type::kDelete, std::move(value)});
 }
 
 FieldValue FieldValueInternal::ServerTimestamp() {
-  return MakePublic(FieldValueInternal{Type::kServerTimestamp,
-                                       Message<google_firestore_v1_Value>{}});
+  Message<google_firestore_v1_Value> value;
+  value->which_value_type = google_firestore_v1_Value_integer_value_tag;
+  value->map_value = {};
+  return MakePublic(
+      FieldValueInternal{Type::kServerTimestamp, std::move(value)});
 }
 
 FieldValue FieldValueInternal::ArrayUnion(std::vector<FieldValue> elements) {