Accept parquet schemas without explicitly required Map keys

[jupiter]

Which issue does this PR close?

Closes #5606.

Rationale for this change

The check is superfluous and restricts reading of a huge number of files produced without Map keys explicitly marked as required.

What changes are included in this PR?

Reduces the check to only error on a value that would have to have been explicitly set to an invalid value.

Are there any user-facing changes?

None found

[tustvold]

Can we add a test for this, I suspect the dremel shredding will not work correctly with just this change.

Perhaps you could get such a file added to the parquet-testing repo so that an integration test can be written

[jupiter]

Ensuring that the key will always be non-nullable in Arrow, and added a test in the module, following the same pattern as other schema conversions.

I can can provide a test file, but not sure how integration tests are written, and I'm not seeing any reference to the test files, other than in the Readme. Should I just add the file 'parquet-testing/data' and add an entry in the Readme?

[tustvold]

https://github.com/apache/parquet-testing/pull/45/files would be a good example

[jupiter]

Also see apache/parquet-testing#47 (comment)

[tustvold]

I would like to see if we can get apache/parquet-testing#47 in first so that we can get a full end-to-end test here. If things drag out though we can move ahead as is

[tustvold]

apache/parquet-testing#47 was merged last week, so I think it should just be a case of updating this PR with an integration test based off that

[jupiter]

How does that look? I've added a test in what I think is an appropriate place, with similar tests. Let me know if there's something specific we should add to the test.

[Jefffrey]

You'll need to bump the relevant submodule too in order to get that new test file

[Jefffrey]

I think just need to fix clippy then should be good to go?

[tustvold]

Thank you for sticking with this 👍