added null safety for llc

packages/stream_chat/lib/src/extensions/map_extension.dart

@@ -1,6 +1,6 @@
 /// Useful extension functions for [Map]
 extension MapX on Map {
   /// Returns a new map with null keys or values removed
-  Map<String, dynamic> get nullProtected =>
-      {...this}..removeWhere((key, value) => key == null || value == null);
+  Map<String, dynamic> get nullProtected => {...this as Map<String, dynamic>}
+    ..removeWhere((key, value) => key == null || value == null);
 }