[11.x] Do not trigger missing translation key handling when checking existence of translation key

[gdebrauwer]

Since #52343, the missing translation key handling is triggered when using the trans_choice method with a translation key that has no translation in the app's current locale but does have a translation in the fallback locale.

// Translation key: "foo.bar"
// Translation in English (en): ":count Foo|:count Bar"
// Translation in Dutch (nl): Not set!
// Fallback Locale is English

app()->setLocale('en');
trans_choice('foo.bar', 1);
// --> Does not trigger "Lang::handleMissingKeysUsing" closure which is correct

app()->setLocale('nl');
trans_choice('foo.bar', 1);
// --> Triggers "Lang::handleMissingKeysUsing" closure which is not correct
//     because a translation in the fallback locale (= 'en') does exist!

This is caused by the fact that it checks the existence of a translation key without a fallback to the fallback locale. To fix this, I changed the has method of the Translator class so that checking the existence of a key does not trigger the Lang::handleMissingKeysUsing closure.