fix: Fix for cells not removed (#115)

There was a logical error in processing `ChildChange` in `Graph`. When cell was removed, `ChildChange` created to change parent of removed cell to `null`. But in code, if parent is null, code to destroy child cell state was not called, so cell stayed rendered on canvas.

The fix add a check for "falsy" parent, so correct branch of code is executed and cells are destroyed.

packages/core/src/view/Graph.ts

@@ -591,8 +591,7 @@ class Graph extends EventSource {
       this.view.invalidate(change.child, true, true);
 
       if (
-        newParent &&
-        (!this.getDataModel().contains(newParent) || newParent.isCollapsed())
+        !newParent || !this.getDataModel().contains(newParent) || newParent.isCollapsed()
       ) {
         this.view.invalidate(change.child, true, true);
         this.removeStateForCell(change.child);