[clang-tidy][readability-braces-around-statements] ignore false-positive for...

[clang-tidy][readability-braces-around-statements] ignore false-positive for constexpr if statement in lambda expression

Fixed: #64545

When TreeTransform, Stmt in constexpr IfStmt will be transform to NullStmt.
This NullStmt has the different beginning token.
This patch add addtional check in checkStmt to handle this case.

Reviewed By: PiotrZSL

Differential Revision: https://reviews.llvm.org/D158480