fix(http): guard against null user in MailboxesController

[ChristophWurst]

For #11224

Error: lib/Controller/MailboxesController.php:73:42: PossiblyNullArgument: Argument 1 of OCA\Mail\Service\AccountService::find cannot be null, possibly null value provided (see https://psalm.dev/078)
Error: lib/Controller/MailboxesController.php:80:38: PossiblyFalseReference: Cannot call method getDelimiter on possibly false value (see https://psalm.dev/105)
Error: lib/Controller/MailboxesController.php:97:45: PossiblyNullArgument: Argument 1 of OCA\Mail\Contracts\IMailManager::getMailbox cannot be null, possibly null value provided (see https://psalm.dev/078)
Error: lib/Controller/MailboxesController.php:139:45: PossiblyNullArgument: Argument 1 of OCA\Mail\Contracts\IMailManager::getMailbox cannot be null, possibly null value provided (see https://psalm.dev/078)
Error: lib/Controller/MailboxesController.php:181:45: PossiblyNullArgument: Argument 1 of OCA\Mail\Contracts\IMailManager::getMailbox cannot be null, possibly null value provided (see https://psalm.dev/078)
Error: lib/Controller/MailboxesController.php:199:45: PossiblyNullArgument: Argument 1 of OCA\Mail\Contracts\IMailManager::getMailbox cannot be null, possibly null value provided (see https://psalm.dev/078)
Error: lib/Controller/MailboxesController.php:219:45: PossiblyNullArgument: Argument 1 of OCA\Mail\Contracts\IMailManager::getMailbox cannot be null, possibly null value provided (see https://psalm.dev/078)
Error: lib/Controller/MailboxesController.php:255:42: PossiblyNullArgument: Argument 1 of OCA\Mail\Service\AccountService::find cannot be null, possibly null value provided (see https://psalm.dev/078)
Error: lib/Controller/MailboxesController.php:271:45: PossiblyNullArgument: Argument 1 of OCA\Mail\Contracts\IMailManager::getMailbox cannot be null, possibly null value provided (see https://psalm.dev/078)
Error: lib/Controller/MailboxesController.php:290:45: PossiblyNullArgument: Argument 1 of OCA\Mail\Contracts\IMailManager::getMailbox cannot be null, possibly null value provided (see https://psalm.dev/078)