return NULL -> return CString() for clarity and efficiency

stepmania/src/DateTime.cpp

@@ -197,7 +197,7 @@ static const CString MONTH_TO_NAME[MONTHS_IN_YEAR] =
 CString MonthToString( int iMonthIndex )
 {
 	if( iMonthIndex < 0 || iMonthIndex >= (int) sizeof(MONTH_TO_NAME) )
-		return NULL;
+		return CString();
 	return MONTH_TO_NAME[iMonthIndex];
 }
 LuaFunction( MonthToString, MonthToString( IArg(1) ) );