Unified Function `Signature` type coercion handling for `Nulls`

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

The code in https://github.com/apache/datafusion/pull/12308 from @mesejo added explicit `DataType::Null` data type handling for type coercion for certain functions

However, this may have introduced a regression for handling Dictionary types https://github.com/apache/datafusion/issues/12670

### Describe the solution you'd like

I would like the "normal" function signature resolution to handle Null coercion rather than requiring functions to provide a custom coerce method (as was done in https://github.com/apache/datafusion/pull/12308)

Comments from https://github.com/apache/datafusion/issues/12670#issuecomment-2385333981

### Describe alternatives you've considered

Ideally I think the coercion logic should be able to substitute `Null` for any data type passed in

So given a signature like
```rust
Exact(vec![Utf8View, Utf8View]),
```

I would expect the coercion logic to be able to handle inputs like the following (by casting `Null` to Utf8View)

```rust
(Null, Null)
(Null, Utf8View)
(Utf8View, Null)
(Utf8View, Utf8View)
```

### Additional context

_No response_

Start by tracing normal function signature resolution and compare it with the custom coercion path introduced by PR #12308, including the Dictionary regression in issue #12670. Validate the listed Exact([Utf8View, Utf8View]) combinations and ensure Null is coerced for every position without regressing Dictionary handling.

Written by the indexing model from the issue text.