handles undefined values from props.onMove

Author: Wayne Van Son

src/react-sortable.tsx

@@ -175,7 +175,9 @@ Please read the updated README.md at https://github.com/SortableJS/react-sortabl
       const { onMove } = this.props;
       const defaultValue = evt.willInsertAfter || -1;
       if (!onMove) return defaultValue;
-      return onMove(evt, originalEvt, this.sortable, store);
+      const result =  onMove(evt, originalEvt, this.sortable, store);
+      if (typeof result === 'undefined') return false
+      return result
     };
 
     return {