fix: Ensure CLOUD key exists before resolving Cloud ID

[enjeck]

Summary

When the Nextcloud server is configured to use an LDAP server whose users are synchronized with the system address book, any user search for sharing a conversation (spreed), a calendar (calendar), or a board (deck) generates exceptions of the form:

"Exception: Argument 1 passed to OC\Federation\CloudIdManager::resolveCloudId() must be of the type string, null given, called in /var/www/nextcloud/lib/private/Collaboration/Collaborators/MailPlugin.php on line 185 in file '/var/www/nextcloud/lib/private/Federation/CloudIdManager.php' line 58"
"Error: Trying to access array offset on value of type null at /var/www/nextcloud/lib/private/Collaboration/Collaborators/MailPlugin.php#185"
"Error: Undefined index: CLOUD at /var/www/nextcloud/lib/private/Collaboration/Collaborators/MailPlugin.php#185"

The line number generating the exception may differ from one Nextcloud version to another.
Each exception is linked to a contact from LDAP. If the search results only contain manually created contacts (present in a personal address book), there are no exceptions.

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)