Explore Updating VariadicAny Signature to take 0 Args

### Is your feature request related to a problem or challenge?

Currently `VariadicAny` represents 1 or more arguments, but given the name and seemingly intention, it seems like 0 or more would make more sense (though there a number of counter examples).

See https://github.com/apache/datafusion/pull/11229/files#r1680888279 for more discussion.

### Describe the solution you'd like

`VariadicAny` is updated to support 0 arguments.

### Describe alternatives you've considered

See the linked discussion for a few alternatives. It's unclear if `VariadicAny` w/ 0 args is the best option here, but was an issue implementing `COUNT()`.

### Additional context

https://github.com/apache/datafusion/blob/b0925c801e1c07bd78c78b045ab58fbd0630b638/datafusion/expr/src/signature.rs#L100-L101

Start with the VariadicAny definition in datafusion/expr/src/signature.rs and read the linked discussion and pull request comments for the alternative designs. Determine whether accepting zero arguments is the intended resolution for the COUNT() implementation, then verify the affected behavior and tests before considering the issue done.

Written by the indexing model from the issue text.