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

include/wx/string.h

@@ -1617,7 +1617,7 @@ public:
     static wxString FromUTF8(const char *utf8)
     {
         if ( !utf8 || !wxStringOperations::IsValidUtf8String(utf8) )
-            return "";
+            return wxString();
 
         return FromImpl(wxStringImpl(utf8));
     }
@@ -1627,7 +1627,7 @@ public:
             return FromUTF8(utf8);
 
         if ( !utf8 || !wxStringOperations::IsValidUtf8String(utf8, len) )
-            return "";
+            return wxString();
 
         return FromImpl(wxStringImpl(utf8, len));
     }