[Translation] MessageCatalogue::add() accepts non-string messages

[rob006]

Symfony version(s) affected

5.4.22

Description

MessageCatalogue::set() expects message to be string, but MessageCatalogue::add() (and the same applies for constructor) does not have any validation for messages array and in practice message could be anything, for example null. This creates inconsistencies, because MessageCatalogue::all() may return these keys with null as value even if MessageCatalogue::has() will return false for them.

In my case loading https://github.com/rob006-software/flarum-ext-last-post-avatar/blob/1ab3d5a5a7b8959f77481e389a1aba08bf97d37f/locale/en.yml resulted rob006-last-post-avatar.forum key with null as value, which resulted deprecation warning in Flarum, because they do not expected that MessageCatalogue::all() may return non-string messages (see rob006-software/flarum-ext-last-post-avatar#2).

How to reproduce

$catalogue = new MessageCatalogue('en');
$catalogue->add(['test' => null]);
assert($catalogue->all() === [] || $catalogue->all(['test' => ''])); 

Possible Solution

I think that all messages should be cast to string before adding to MessageCatalogue::$messages. I case of loading YAML like https://github.com/rob006-software/flarum-ext-last-post-avatar/blob/1ab3d5a5a7b8959f77481e389a1aba08bf97d37f/locale/en.yml I would expect that these null-keys will be completely skipped, but I'm not sure if skipping them in MessageCatalogue::add() would be a good place to do this.

Additional Context

No response