Locale parameter in MessageSource#getMessage methods should be nullable

[dougbreaux]

My IDE is complaining that I'm passing a null Locale value to:

String getMessage(String code, @Nullable Object[] args, Locale locale) throws NoSuchMessageException;

Which obviously does not have @Nullable, but the implementation definitely accepts null and uses the default Locale, which is what we want and have been doing with Spring 5.x.

And, in fact, AbstractMessageSource.getMessageInternal does have @Nullable:

protected String getMessageInternal(@Nullable String code, @Nullable Object[] args, @Nullable Locale locale) {