Changes proposed in this pull request:

Scenario: User logins with OAuth2, the OAuth server provides and numeric id and a string nickname such as "Robert Lan". This nickname is not valid as username in Flarum and will fail the regex validation. Hence I wish to set the user's name as nickname as it can't be a username.

Purpose of change: Today the Nickname signup modal doesn't respect code to provide nickname when username is also supplied.

Example from OAuth call
->provide('username', $user->getId())
->provide('nickname', $user->getName())

This ends up with both nickname and username fields just replacing the username. This is because of this line

What has changed
I've just added a check for attr.nickname before it falls back to username or blank. This seems to solve the issue.

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)