[2D] [Animation] Clearing All Categories for all sprites on a character does not delete the Sprite Lib

Steps to Repro:

1. import a character PSD and open the Skinning Editor
2. create/assign Category for sprites to create Sprite Lib for the character
3. clear all Categories for all sprites and Apply

Result: An empty Sprite Lib remains for the character
Expected: The Sprite Lib is deleted as it is no longer needed/used