Value of Fediverse account field is not sanity checked

[nickvergessen]

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A colleague on our instance has set the fediverse field to Firstname Lastname, yielding the following error when you visit their profile: Undefined array key 1 at …/lib/private/Profile/Actions/FediverseAction.php#81

The problem is that https://github.com/nextcloud/server/blob/aa5f037af71c915424c6dcfd5ad2dc82797dc0d6/lib/private/Profile/Actions/FediverseAction.php#L81 just assumes it is an "Email alike" value.

We should add some sanity check there and also when adding a value.

[hanwick]

Interested in tackling this issue.