[clang-tools-extra] [clang][dataflow]Use cast_or_null instead of cast to prevent crash (PR #68510)

[Yitzhak Mandelbaum via cfe-commits]

https://github.com/ymand approved this pull request.

Thanks, looks good! 

You can submit as is, but if you're up for it, it would actually be better to add the new test case directly to the model's unittests. Something like this test (though just one case is enough -- please put it in a separate TEST_P): https://github.com/llvm/llvm-project/blob/a3a0f59a1e1cb0ac02f06b19f730ea05a6541c96/clang/unittests/Analysis/FlowSensitive/UncheckedOptionalAccessModelTest.cpp#L1966

In general, the lit tests for the clang tidy check only do minimal testing to ensure the check is properly calling the model.

https://github.com/llvm/llvm-project/pull/68510