[clang] [clang] Add additional nullptr guards to StaticAnalyzer (PR #210823)

[Ian Li via cfe-commits]

ianayl wrote:

> Please adjust the commit title and summary before merging. Right now, the phrasing strongly suggests that there was a problem that it's fixing - while in fact, this is speculation at best.

I've fixed the wording here, let me know if this is good.

I don't actually have merge permissions yet, if this looks good to you could you help me merge as well? Thanks in advance!

https://github.com/llvm/llvm-project/pull/210823