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/translation.cpp

@@ -1456,7 +1456,7 @@ void wxTranslations::SetLoader(wxTranslationsLoader *loader)
 void wxTranslations::SetLanguage(wxLanguage lang)
 {
     if ( lang == wxLANGUAGE_DEFAULT )
-        SetLanguage("");
+        SetLanguage(wxString());
     else
         SetLanguage(wxLocale::GetLanguageCanonicalName(lang));
 }
@@ -1926,7 +1926,7 @@ wxArrayString wxFileTranslationsLoader::GetAvailableTranslations(const wxString&
             continue;
 
         wxString lang;
-        for ( bool ok = dir.GetFirst(&lang, "", wxDIR_DIRS);
+        for ( bool ok = dir.GetFirst(&lang, wxString(), wxDIR_DIRS);
               ok;
               ok = dir.GetNext(&lang) )
         {