fix: null as 2nd param in preg_match is deprecated

[imorland]

**Replaces #3783 **

Changes proposed in this pull request:
Deprecated: preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated in /var/www/framework/framework/core/src/Locale/Translator.php on line 65

Reviewers should focus on:

Screenshot

QA

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)