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

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 = "");

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18829