Special-case `nullif` kernel for scalar using `Datum`

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

In https://github.com/apache/datafusion/pull/11697 we have to convert a scalar lhs of `nullif` to an array of the same size as rhs, since `nullif` does not support the `Datum` trait.

I think it would be faster if `nullif` could directly work with scalar arrays.

**Describe the solution you'd like**

Change the type of lhs of `nullif` to be `Datum`

**Describe alternatives you've considered**

**Additional context**

Review the linked DataFusion PR 11697 first, then locate the nullif kernel and its Datum-related call path in arrow-rs. Done means a scalar lhs can be handled directly as a Datum instead of being converted to an rhs-sized array, while preserving the existing nullif behavior.

Written by the indexing model from the issue text.