The “+” button to create Labels is not deactivated when no labels can be created due to no Categories existing and does nothing when pressed

Steps to reproduce:

1. Create a new Unity project using the Universal 2D template
2. Open the Sprite Library Editor window (Window > 2D > Sprite Library Editor window)
3. Dock the Sprite Library Editor window
4. Click the "Create new Sprite Library Asset"
5. Click the "+" button next to Labels
6. Observe the Sprite Library Editor window

Actual results: The "" button is not deactivated even though it cannot create a new label and nothing happens when the "" button is pressed

Expected results: The "+" button is deactivated until a category is created

Reproducible with versions: 6000.0.67f1, 6000.3.9f1, 6000.4.0b9, 6000.5.0a7

Tested on (OS): MacOS Silicon Tahoe 26.2, Ubuntu 24.04