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

[Balazs Benics via cfe-commits]

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

You are right.
I'm not sure it improves the code too much, and I wonder if you have further ideas refactoring the checker; if so we could probably bundle up similar changes into this one.

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