Clear sub-resources list when no sub-resource exists

[timothyqiu]

This PR makes sure that the sub-resources list in "Make Sub-Resources Unique" dialog is cleared when no resource is available.

Previously, the issue can be reproduced by:

1. Create a Sprite2D node.
2. Open "Make Sub-Resources Unique" dialog from the inspector. Nothing is in the list.
3. Drag icon.svg into Texture.
4. Open the dialog again. You can see texture is in the list.
5. Clear the Texture property.
6. Open the dialog again. texture is still in the list, but the description says "This object has no resources".

[YuriSizov]

Thanks!

[YuriSizov]

Cherry-picked for 4.2.2.