Fix parameter type checking for unassigned/optional arguments

[Inconnu08]

Fixes an issue in check_parameter_types where unassigned parameters
with type annotations could trigger a panic.

Previously:

• None values from get_slot caused a panic
• This broke the expected behavior for optional or unassigned parameters

Now:

• Unassigned parameters are handled gracefully
• Type checking is only applied when a value is present

All relevant tests pass.

Resolves #182

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[meta-codesync]

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