[clang][dataflow][NFC] Fix outdated comment on getStableStorageLocation

Follow-up to D129097.

It is no longer a requirement that the `QualType` passed to to
`DataflowAnalysisContext::getStableStorageLocation()` is not null. A
null type pass as an argument is only applicable as the pointee type
of a `std::nullptr_t` pointer.

Differential Revision: https://reviews.llvm.org/D131109