Allow nullable PKs

[nirinchev]

Fixes #458

[coveralls]

Pull Request Test Coverage Report for Build 2102883118

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.4%) to 91.469%

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Files with Coverage Reduction	New Missed Lines	%
generator/lib/src/utils.dart	1	66.67%

Totals
Change from base Build 2094856042:	-0.4%
Covered Lines:	386
Relevant Lines:	422

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💛 - Coveralls

[blagoev]

I would like to have a discussion about this offline.

[nirinchev]

@blagoev did you get a chance to get buy-in from @ianpward on not supporting nullable PKs in the dart SDK? If so, I propose we close the PR and we can always reopen if user requests come in.

[blagoev]

I converted to draft. If we close it someone might delete the branch. Let's keep it for a while. I know dangling PRs are not great but...

[nirinchev]

We can always restore the branch from the Github UI. And by the time we decide to work on it, the code will likely have rotted enough, that it'd be easier to just use the PR as inspiration and start from scratch.