test(ldap): return empty string for unset ldapUserDisplayName2 as per implementation

[joshtrichards]

• Resolves: #

Summary

Fixes this test warning with PHP 8.5:

7) /home/runner/actions-runner/_work/server/server/apps/user_ldap/lib/Access.php:706
Using null as an array offset is deprecated, use an empty string instead

Triggered by:

* OCA\User_LDAP\Tests\AccessTest::testUserStateUpdate
  /home/runner/actions-runner/_work/server/server/apps/user_ldap/tests/AccessTest.php:711

TODO

• ...

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI

[joshtrichards]

/backport to stable33