[clang-tools-extra] [clang-tidy] Add null-checking in Use designated Initializer check (PR #220093)

[via cfe-commits]

GTaf wrote:

@vbvictor if you have time to review this PR, it's ready 
Also, I believe this bug is a bit cosmetic, but does not fix the root issue which is having a null Node. Am I correct to believe another fix (to be done in the semantic analysis part) should be done in another PR ?

https://github.com/llvm/llvm-project/pull/220093