add Throwable.getCause to nullableReturns

[XN137]

reasoning:

• in its default implementation getLocalizedMessage() calls getMessage(), so should be treated in the same way
• not every throwable was created with a cause, so getCause() will often return null, as indicated by its javadoc

[CLAassistant]

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[msridhar]

LGTM, thanks for the contribution! This will cause NullAway to report new errors in the next release but I think it's worth it. @lazaroclapp PTAL in case I missed something. I'll hold off on landing until then. @XN137 may take a couple of days due to other deadlines

[lazaroclapp]

Makes sense to me. Thanks for the contribution! 🚀

Agree with @msridhar this will likely cause new errors for existing NullAway enrolled codebases, but a) they very well could be true positives, b) we can always (say, internally, at Uber) override these models for a bit if we need to.