[clang] [clang][analyzer]][NFC] Simplify method 'ensureStreamNonNull' of StreamChecker (PR #70927)

[Ben Shi via cfe-commits]

benshi001 wrote:

> much, and I wonder if you have further ideas refactoring the checker; if so we could probably bundle up similar changes into this one.

Thanks for your reply. Actually I have no plan about improving this checker. Currently I am just reading its code and trying to understand its details. 

https://github.com/llvm/llvm-project/pull/70927