fix: allow null in User::setIdentities()

[wakqasahmed]

Description

Fixes #1302

$identities is ?array and null is what tells getIdentities() to reload from the database, but setIdentities() only accepted array. So in a subclass of User, where the private property isn't reachable, $this->identities = null goes through the entity setter and throws a TypeError. Same for $user->identities = null from outside.

As discussed in the issue, this changes the signature to setIdentities(?array $identities), and adds a test that setting identities to null makes the next read pick up a newly added identity.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide