cube map textures error if not set

Steps to reproduce:

WebGPU requires a texture to be bound if the shader expects a texture. A dummy texture will be used if one isn't set explicitly. There is an error if the shader expects a cube texture, as a 2d texture will be bound. WebGPU should provide a dummy cube texture in this scenario.

Actual results: 

Expected results: 

Reproducible with versions: 

Not reproducible with versions: 

Can’t test with versions: 

Tested on (OS): 

Notes:

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