fix: Check generation for nullability before incrementing it

[domrde]

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Fixes #886 ☕️

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[domrde]

Hey, @BenWhitehead, am I doing something wrong? Why clirr is failing?

[BenWhitehead]

@domrde I'm not sure why it's failing. It looks like some transient failure downloading jars during the build.

I've manually ran clirr against this PR and verified it does not error. I'll merge shortly.