Alter the wxString gdb pretty-printer to improve its output for empty wxStrings

Gdb versions >=7.6 were outputting a very long and unhelpful result when inspecting an empty wxString e.g.
0x7ffff55d04b8 <std::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t> >::_Rep::_S_empty_rep_storage+24> "" [wxString]
Changing to the method used in the official wx pretty-printer fixes this, without breaking gdb 7.4.1 or 7.5. The only slight downside is that the string's address is no longer output.