Avoid unnecessary calls to String#format()

Generated message bundle methods always contain an invocation of String#format() also if there actually is no parameter to be replaced, e.g. like this:

@Override
public final String mustNotBeNull() {
    String result = String.format(mustNotBeNull0$str());
    return result;
}

AFAICS the invocation of format() seems superfluous, instead the String might directly be returned for performance reasons.