Fix mutable default arguments in OrderingFilter methods

[killerdevildog]

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Description

• Fixed get_default_valid_fields() and get_valid_fields() methods in filters.py
• Changed context={} default parameter to context=None to prevent mutable default anti-pattern
• Added proper None checking with context = {} assignment inside methods

Why this fix is important:

• Mutable default arguments (context={}) create shared state across function calls
• Same dict object gets reused, potentially causing unexpected side effects
• This is a well-known Python anti-pattern that can lead to bugs

What was changed:

• Line 249: get_default_valid_fields(self, queryset, view, context=None)
• Line 285: get_valid_fields(self, queryset, view, context=None)
• Added 'if context is None: context = {}' in both methods

Testing results:

• All existing filter tests pass (pytest tests/test_filters.py)
• Custom verification script confirms fix works correctly
• Maintains backward compatibility
• No breaking changes to API

Fix #9741

[browniebroke]

Due to how filter are used, I don't think that's ever a problem in practice (we seem to always pass the context), but shouldn't hurt to fix anyway. The only thing that could break is if None is valid value for context, and the user explicitly passes None. In which case, this would override it with an empty dict {}. Can't think of a case where that would be a problem though...

Fix looks good to me, but would be nice to add a test or 2 to make sure this doesn't regress.

Thanks

[killerdevildog]

@browniebroke I'll look into this this week.