Empty string for texture reference in model breaks it

If a model file has a texture with its value set to the empty string, the model fails to load. This is possibly intended, though there's no specific error message for this, it just throws a Java error.

How to reproduce

• Download and equip the attached pack
  In 22w42a, the pack cannot be equipped due to MC-256854

Code

It's caused by BlockModel#isTextureReference, which checks the first character of the string without checking its length