Consider adding checks that validate eq(null) / notEq(null) is not used on a required column in the filter2 API

Is this too strict?

**Reporter**: [Alex Levenson](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=alexlevenson) / @isnotinvain

_{**Note**: *This issue was originally created as [PARQUET-44](https://issues.apache.org/jira/browse/PARQUET-44). Please see the [migration documentation](https://issues.apache.org/jira/browse/PARQUET-2502) for further details.*}

Start by locating the filter2 API handling for eq(null) and notEq(null), along with how required columns are represented. Clarify whether rejecting these cases is intended, then define tests that establish the accepted behavior for required columns before implementing any validation.

Written by the indexing model from the issue text.