Re-support Signature::Any(0), and add tests for zero argument udfs

[alamb]

Which issue does this PR close?

• Closes Make migration to Signature::nullary in 44.0.0 easier / less confusing #13763

Rationale for this change

As described on #13763

In version s 43.0.0 and earlier of DataFusion, a TypeSignature::Any(0) means your udf could be called with zero arguments:

SELECT my_udf();

However in version 44.0.0 you get a very confusing error that says zero arguments are not supported but then gives you a candidate function is the one it just said was not supported:

SELECT my_udf();

Error during planning: example_union does not support zero arguments. 
No function matches the given name and argument types 'my_udf()'. 
You might need to add explicit type casts.

Candidate functions:
example_union()

I believe @jayzhan211 added this error to try and guide people to ensure people used the newly introduced Signature::Nullary (see #13763 (comment))

What changes are included in this PR?

1. Add tests showing various ways to call udfs with no arguments
2. Update Signature::supports_zero_args

Are these changes tested?

Are there any user-facing changes?

[alamb]

I actually think we could consider removing this function entirely (perhaps deprecate) as it is only used to generate errors

[jayzhan211]

I update the fix here alamb#22