[clang-tidy] readability-container-size-empty handle std::string length()

Extends readability-container-size-empty to check std::string length() similar to size().

Fixes: #37603

Co-authored-by: Dmitry Venikov <quolyk@gmail.com>

Reviewed By: carlosgalvezp

Differential Revision: https://reviews.llvm.org/D56644