Use wxString() instead of "" for empty strings

This will allow this code to work even when implicit conversion from
"const char*" is disabled in wxString and is already marginally more
efficient even now.

See https://github.com/wxWidgets/wxWidgets/pull/782

src/common/filename.cpp

@@ -2464,7 +2464,7 @@ void wxFileName::SplitPath(const wxString& fullpath,
 wxString wxFileName::StripExtension(const wxString& fullpath)
 {
     wxFileName fn(fullpath);
-    fn.SetExt("");
+    fn.SetExt(wxString());
     return fn.GetFullPath();
 }