Reduce the number of subcases in null_bind_group_layouts

[kainino0x]

Please check to make sure you still think this provides sufficient test coverage.

Before, with null+undefined: 310 subcases
With just null: 90 subcases
With this change: 30 subcases

INFO: subcase: bindGroupLayouts=["Null"]
INFO: subcase: bindGroupLayouts=["Undefined"]
INFO: subcase: bindGroupLayouts=["Null","NonEmpty"]
INFO: subcase: bindGroupLayouts=["Undefined","NonEmpty"]
INFO: subcase: bindGroupLayouts=["NonEmpty","Null"]
INFO: subcase: bindGroupLayouts=["NonEmpty","Undefined"]
INFO: subcase: bindGroupLayouts=["Null","NonEmpty","NonEmpty"]
INFO: subcase: bindGroupLayouts=["Undefined","NonEmpty","NonEmpty"]
INFO: subcase: bindGroupLayouts=["NonEmpty","Null","NonEmpty"]
INFO: subcase: bindGroupLayouts=["NonEmpty","Undefined","NonEmpty"]
INFO: subcase: bindGroupLayouts=["NonEmpty","NonEmpty","Null"]
INFO: subcase: bindGroupLayouts=["NonEmpty","NonEmpty","Undefined"]
INFO: subcase: bindGroupLayouts=["Null","NonEmpty","NonEmpty","NonEmpty"]
INFO: subcase: bindGroupLayouts=["Undefined","NonEmpty","NonEmpty","NonEmpty"]
INFO: subcase: bindGroupLayouts=["NonEmpty","Null","NonEmpty","NonEmpty"]
INFO: subcase: bindGroupLayouts=["NonEmpty","Undefined","NonEmpty","NonEmpty"]
INFO: subcase: bindGroupLayouts=["NonEmpty","NonEmpty","Null","NonEmpty"]
INFO: subcase: bindGroupLayouts=["NonEmpty","NonEmpty","Undefined","NonEmpty"]
INFO: subcase: bindGroupLayouts=["NonEmpty","NonEmpty","NonEmpty","Null"]
INFO: subcase: bindGroupLayouts=["NonEmpty","NonEmpty","NonEmpty","Undefined"]
INFO: subcase: bindGroupLayouts=["Empty"]
INFO: subcase: bindGroupLayouts=["Empty","NonEmpty"]
INFO: subcase: bindGroupLayouts=["NonEmpty","Empty"]
INFO: subcase: bindGroupLayouts=["Empty","NonEmpty","NonEmpty"]
INFO: subcase: bindGroupLayouts=["NonEmpty","Empty","NonEmpty"]
INFO: subcase: bindGroupLayouts=["NonEmpty","NonEmpty","Empty"]
INFO: subcase: bindGroupLayouts=["Empty","NonEmpty","NonEmpty","NonEmpty"]
INFO: subcase: bindGroupLayouts=["NonEmpty","Empty","NonEmpty","NonEmpty"]
INFO: subcase: bindGroupLayouts=["NonEmpty","NonEmpty","Empty","NonEmpty"]
INFO: subcase: bindGroupLayouts=["NonEmpty","NonEmpty","NonEmpty","Empty"]

Issue: #4075, follow-up to #4076

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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.