[2D] [Sprite Mask] Material is created when dragging a sprite into the Inspector view of a Sprite Mask

Steps to Repro:

1. create a new Sprite Mask in the Hierarchy
2. create/import a Sprite into Assets
3. select the Sprite Mask in Hierarchy
4. drag the Sprite into the Inspector of the Sprite Mask

Result: A new Material is created for the Sprite.
Expected: No Material is created.