Use empty() member function to determine if string is empty

Use this dedicated function instead of checking if length() function returns zero/non-zero value.

include/wx/combo.h

@@ -482,8 +482,8 @@ protected:
     {
         return ( !m_text &&
                  !(flags & wxCONTROL_ISSUBMENU) &&
-                 !m_valueString.length() &&
+                 m_valueString.empty() &&
-                 m_hintText.length() &&
+                 !m_hintText.empty() &&
                  !ShouldDrawFocus() );
     }