Add validation test about creating pipeline layout with null bind group layout

[Jiawei-Shao]

Issue: #4075

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Requirements for PR author:

• [*] All missing test coverage is tracked with "TODO" or .unimplemented().
• [*] New helpers are /** documented */ and new helper files are found in helper_index.txt.
• [*] Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)
• [*] Test have be tested with compatibility mode validation enabled and behave as expected. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located in the test tree.
• Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Helpers and types promote readability and maintainability.

When landing this PR, be sure to make any necessary issue status updates.

[Jiawei-Shao]

Hi @jzm-intel and @kainino0x ,

With latest webgpu/types I cannot test undefined as a member of GPUPipelineLayoutDescriptor.bindGroupLayouts so I only test null in this PR. I've raised this issue as gpuweb/types#162.

PTAL, thanks!

[kainino0x]

Optional nits but feel free to land with or without!

[kainino0x]

optional nit: I think the filter can be a bit simpler if you start out like this:

Suggested change

	.combine('bindGroupLayout1', MaybeNullBindGroupLayoutTypes)
	.combine('bindGroupLayout2', MaybeNullBindGroupLayoutTypes)
	.combine('bindGroupLayout3', MaybeNullBindGroupLayoutTypes)
	.expand('bindGroupLayout1', p => p.bindGroupLayoutCount > 1 ? MaybeNullBindGroupLayoutTypes : ['Null'])
	.expand('bindGroupLayout2', p => p.bindGroupLayoutCount > 2 ? MaybeNullBindGroupLayoutTypes : ['Null'])
	.expand('bindGroupLayout3', p => p.bindGroupLayoutCount > 3 ? MaybeNullBindGroupLayoutTypes : ['Null'])

[kainino0x]

Maybe:

// Only test cases where at least one of the bind group layouts is null.
const allBGLs = [p.bindGroupLayout0, p.bindGroupLayout1, p.bindGroupLayout2, p.bindGroupLayout3];
const bgls = allBGLs.slice(0, p.bindGroupLayoutCount);
return bgls.includes('Null');