Actually fix GH-9583

The issue is that PS(mod)->s_validate_sid is always defined for user modules, thus we need to check that the actual callable is set
Add another regression test to en

Actually fix GH-9583

The issue is that PS(mod)->s_validate_sid is always defined for user modules, thus we need to check that the actual callable is set
Add another regression test to ensure current working behaviour is not broken (which was by the previous incorrect fix)

Closes GH-9638

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