[clang-tidy] Fix segfault in bugprone-standalone-empty

The check incorrectly identified empty() method call in the template
class definition as a stand-alone function call.
This led to a crash because the checker did not expect empty() function
calls without arguments.

Fixes: https://github.com/llvm/llvm-project/issues/59487

Reviewed By: cjdb

Differential Revision: https://reviews.llvm.org/D142423