[clang-tidy] Fix FP with readability-redundant-string-init for default arguments

[etienneb]

Summary

Clang-tidy is reporting a warning of redundant string initialisation
on a string parameter initialized with empty string.

See bug: 27087

The reported example is:

#include <string>
void fn(std::string a = "");

[alexfh]

Awesome! Thanks for the fix!