fix: handle None module in module_available

[jaythehardcoder]

Description

Closes #11344.

module_available(None) crashes with AttributeError: 'NoneType' object has no attribute 'startswith' during version checks (e.g., in the rain-to-flood toolkit).

This adds an early return of False when module is None, and a regression test covering None input, valid modules, and minversion checks.

Checklist

• Closes Load failure (in rain to flood toolkit) #11344
• Tests added
• User visible changes (including notable bug fixes) are documented in whats-new.rst
• New functions/methods are listed in api.rst

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
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    Tools: Hermes Agent

Test Plan

• uv run pytest xarray/tests/test_namedarray.py::test_module_available -v -> 1 passed
• uv run pytest xarray/tests/test_namedarray.py -q -> all existing tests still pass

[Illviljan]

How do we get a None as input? This function does not allow anything else than str.
Is this an actual problem in xarray?

[jaythehardcoder]

It happens when external tooling (like uv scripts / pip-run workflows) imports xarray. In some Python environments, importlib.metadata.entry_points() returns None for a specific group, which gets passed into module_available() as the module parameter — see issue #11344 for the exact traceback. The fix handles that edge case gracefully instead of crashing.