lang/funcs: type conversion functions can convert null values

[apparentlymart]

We had intended these functions to attempt to convert any given value, but there is a special behavior in the function system where functions must opt in to being able to handle dynamically-typed arguments so that we don't need to repeat the special case for that inside every function implementation.

In this case we do want to specially handle dynamically-typed values, because the keyword null in HCL produces
cty.NullVal(cty.DynamicPseudoType) and we want the conversion function to convert it to a null of a more specific type.

These conversion functions are already just a thin wrapper around the underlying type conversion functionality anyway, and that already supports converting dynamic-typed values in the expected way, so we can just opt in to allowing dynamically-typed values and let the conversion functionality do the expected work.

Fixing this allows module authors to use type conversion functions to give additional type information to Terraform in situations that are too ambiguous to be handled automatically by the type inference/unification process. Previously tostring(null) was effectively a no-op, totally ignoring the author's request to treat the null as a string.

This closes #30866, by providing a way to annotate the type-ambiguous input with more specific type information so that type unification can succeed.

[github-actions]

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[github-actions]

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