Require all zero argument UDFs use Signature::Nullary, improve error messages

[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()

Also, Signature::Exact(vec![]) does work.

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 to disallow exact

Are these changes tested?

Yes, now integration tests are added, and redundant unit tests removed

Are there any user-facing changes?

[jayzhan211]

I update the fix here alamb#22

[alamb]

Given there are a few more built in functions with exact, I think we should postpone this PR until next release (aka postpone merge for a few days)

In the intervening time I will prepare a PR that just improves the error message for 43.

Thank you for your help @jayzhan211

[alamb]

Here is a PR with just improvements to the error messages

• Improve error messages for incorrect zero argument signatures #13881

[alamb]

I don't have time to finish this work now. If anyone else wants to try and make progress, feel free