[1.x] fix(core, mentions): return null if content left empty in formatter

[DavideIadeluca]

Fixes #3983

Changes proposed in this pull request:

• Return null instead of void if content field left empty while creating new discussion
• Remove typings from mentions Unparser and change type annotation to mixed
• Implement test to cover the exact case that was described in the linked issue

Reviewers should focus on:

• Is this change backwards compatible? Might bis break third-party extensions which used workarounds to fix this issue?
• Will PR 2.x in the next coming days for the same fix

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)