[code style] com_messages backend

I think we need 2 tests on this because changes done in:
https://github.com/phproberto/joomla-cms/compare/joomla:staging...phproberto:cs-com_messages-backend?expand=1#diff-58d47869bc03e4659c4fa82975614951R28

It fixes the issues about params with no default values at the end of the parameter list on function definition. Same fix we applied in:

https://github.com/joomla/joomla-cms/blob/staging/libraries/cms/html/select.php#L157-L160

So the param is defined as optional but internally it's still required to ensure B/C
@phproberto Missing since tag.