Remove unnecessary null checks in `GroupColumn`s

[Rachelint]

Is your feature request related to a problem or challenge?

As mentioned by @Dandandan in https://github.com/apache/datafusion/pull/12809#discussion_r1801678804

Some null checks are actullay unnecessary for arrays containing no nulls (basically we can just use null_count to check it).

Describe the solution you'd like

As mentioned above, the simple way is using null_count in array to make it.

Furtherly, I found we indeed check which rows are nulls in create_hashes. I think maybe we can reuse this result?

Describe alternatives you've considered

No response

Additional context

No response