Consider changing nullability of UDFs to depend on inputs by default

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

See: https://github.com/apache/datafusion/pull/19213#issuecomment-3635519486

Currently nullability is true by default for all UDFs; consider making it depend on input nullabilities (only nullable if inputs are nullable) since that looks to be the more common case. Exception functions would implement it back to true if this behaviour is not desirable.

This is likely to be a notable breaking change so discussion is needed first and the changes would need to be made with care.

Also related:

### Describe the solution you'd like

_No response_

### Describe alternatives you've considered

_No response_

### Additional context

Related:

- https://github.com/apache/datafusion/issues/18882

Start by reading the linked pull request comment and related issue 18882 to understand the proposed UDF nullability behavior and existing concerns. The work is not ready for implementation until the breaking-change discussion produces an agreed design; done would include that decision and a carefully scoped implementation plan.

Written by the indexing model from the issue text.