fix: prevent unwarranted a11y warnings on custom Button subclasses

[davwheat]

Changes proposed in this pull request:

UploadImageButton has its own content in the view method, so we don't need the empty children a11y warning.

Reviewers should focus on:

N/A

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.
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.