fix: isDark() utility can receive null value

[exside]

Fixes #3764

Changes proposed in this pull request:
Shouldn't be many, was mainly an edge case when there were NULL values in the db that would crash this method.

Necessity

• Has the problem that is being solved here been clearly explained?
  Well, it doesn't crash anymore =)
• If applicable, have various options for solving this problem been considered?
  Decided to not make the method too fancypants for performance reasons (e.g. no regex checks for valid hex colors etc.)
• 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)

[SychO9]

Thanks for the PR!