GH-586: Override fixedSizeBinary method for UnionMapWriter

[axreldable]

What's Changed

UnionMapWriter will null out the entire map struct entry instead of setting the value to null in:

childWriter.value().fixedSizeBinary().writeNull();

This PR overrides the fixedSizeBinary method for the UnionMapWriter, resolving it.

Closes #586.

[axreldable]

Thank you for opening a pull request!

Please label the PR with one or more of:

• bug-fix
• chore
• dependencies
• documentation
• enhancement

Also, add the 'breaking-change' label if appropriate.

See CONTRIBUTING.md for details.

Hm, looks like I don't have enough permissions to add labels.

Could the maintainers add the bug-fix label, please?

[axreldable]

Hi @nbauernfeind! It's my attempt to address the issue you opened. Could you please take a look? Is it what you had in mind for it?

[axreldable]

Hi @lidavidm , @jbonofre! Could you please check this? Or at least add the bug-fix label to ensure PR format to release CI build. Thank you!

[jbonofre]

@axreldable sure. I will. FYI we are working on fixing the CI right now. Your PR will probably need a rebase as soon as CI is green again. I will keep you posted.