fix: error in funding composer.json block bricks frontend

[davwheat]

Changes proposed in this pull request:

Any error in composer.json's funding block syntax will brick the admin frontend.

Oddly, this error seems to persist after correcting the error, composer update and a cache clear on my local dev forum. I had to implement this to allow me to toggle the extension, which then fixed the error.

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.
• 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.

[askvortsov1]

Instead of this, could we use something like Arr::get($funding, '0.url')?

[davwheat]

That was a fair bit simpler.