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

[Qizhi Hu via cfe-commits]

jcsxky wrote:

> Thanks for this fix! Unfortunately, I wasn't able to repro the crash in godbolt: https://godbolt.org/z/s741z5djY. Can you double check that the check crashes on that example without your fix?

The test case is different from that in this patch. Use `float` in `optional`.

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