Minor: Add tests showing aggregate behavior for NaNs

[alamb]

Which issue does this PR close?

Related to https://github.com/apache/datafusion/pull/10627 and https://github.com/apache/datafusion/issues/8031

Rationale for this change

While looking at https://github.com/apache/datafusion/pull/10627 from @westonpace I couldn't find any existing tests for how Nans worked in aggregates.

What changes are included in this PR?

Add a test that documents how Nans work today in the basic aggregates

Are these changes tested?

Only tests

Are there any user-facing changes?

No