fix NPE on commit path when the last persistent iterator is removed (#…

…772)

Co-authored-by: Badrish Chandramouli <badrishc@microsoft.com>

cs/src/core/FasterLog/FasterLog.cs

@@ -1771,7 +1771,7 @@ private bool IteratorsChanged(ref FasterLogRecoveryInfo info)
             {
                 return info.Iterators != null &&  info.Iterators.Count != 0;
             }
-            if (_lastPersistedIterators.Count != info.Iterators.Count)
+            if (info.Iterators == null || _lastPersistedIterators.Count != info.Iterators.Count)
                 return true;
             foreach (var item in _lastPersistedIterators)
             {