[snapping] Invalidate the filtered layer tree model when a parent group switches from empty/non-empty state #59668
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Description
This PR fixes #59469 whereas creating a new group and moving layers into that group doesn't show in the advanced snapping layer tree widget.
The layers are hidden as the snapping layer tree model filters out empty group. When children are added into the group, the filtered model didn't invalidate its filter, resulting in both the group and the added children gone missing from the tree widget.