Pass null/falsy values through Model.getIdentifier

[dsevillamartin]

Fixes #2876

Changes proposed in this pull request:

• Return model in Model.getIdentifier if falsy

Reviewers should focus on:

• Do we just want to handle null or all falsy values?
  • I think handling all is better as they're still visible in the request data, so should be easy to debug
  • I also think we should pass through the actual value instead of returning null, so that we don't modify the data (would add confusion)
• Do we want to handle falsy/non-object model.data?
  • Personally I don't think so, as that would probably be an actual bug and not intentional. Having the error appear is probably the preferred situation.
• Do we want an if statement instead to make the code clearer?
• Do we want to handle this in getIdentifier method?

Screenshots
Before

After

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]

I think I would slightly prefer an explicit if-return for falsy model instances, but that's minor. Otherwise LGTM!