[FLINK-3842] [table] Fix handling null record/row in generated code

[twalthr]

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This PR adds support for nullable rows. It will be tested by FLINK-2971. Manual tests passed.

[fhueske]

Thanks @twalthr, the PR looks good. Will merge it after the tests pass.

[dawidwys]

I think those failing tests are unrelated.

[fhueske]

Yes, you are right @dawidwys.
I'm usually batching several PRs and trigger another build just to be on the safe side ;-)