Update parse_array.dart

This fixes the issue because the **_handleSingleResult()** function which handles the response for a single parse object in the **_ParseResponseBuilder** was calling **_notifyChildrenAboutSave();** so when **save()** was called on any object it called **_notifyChildrenAboutSaving();** first and then in the response when **_notifyChildrenAboutSave();** was called, the **_estimatedArrayBeforeSaving** had values and _savedArray was not set as empty.
But in case of other queries where single object was returned like **getUpdatedUser()** or **fetch()** on any object it just called **_notifyChildrenAboutSave();** in the **_handleSingleResult()** function which resulted in **_savedArray** being set as empty since **_estimatedArrayBeforeSaving** was null. I just added the null safety and not empty check before setting the **_savedArray** to **_estimatedArrayBeforeSaving** so now the savedArray is never returned empty in the response and the issue is resolved.

Author: nabos440

packages/dart/lib/src/objects/parse_array.dart

@@ -68,9 +68,11 @@ class _ParseArray implements _Valuable<List>, _ParseSaveStateAwareChild {
   @mustCallSuper
   void onSaved() {
     setMode = false;
+    if(_estimatedArrayBeforeSaving != null){
     _savedArray.clear();
-    _savedArray.addAll(_estimatedArrayBeforeSaving ?? []);
+    _savedArray.addAll(_estimatedArrayBeforeSaving!);
     _estimatedArrayBeforeSaving = null;
+    }
 
     if (_lastPreformedOperationBeforeSaving == lastPreformedOperation) {
       // No operations were performed during the save process