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

[Piotr Zegar via cfe-commits]

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

LGTM,
When release notes could be nice (clang-tools-extra/doc/ReleaseNotest.rst) with something like: `Improved bugprone-unchecked-optional-acces check to not crash during handling of optional values` or `to not crash i certain situations`  it may also not be so necessary, as this is just very quick fix.

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