Fix some PHP8 errors for IMAP backend

[DoobleD]

Released under the GNU Affero General Public License (AGPL), version 3 and Trademark Additional Terms.

What does this implement/fix? Explain your changes.

Fixes some errors with PHP 8.x for the IMAP backend. In particular this change line 1131 fixes these 2 fatal errors:

[FATAL] [<user>] Fatal error: /<path>/z-push/backend/imap/imap.php:1132 - Uncaught TypeError: implode(): Argument #2 ($array) must be of type ?array, bool given in /<path>/z-push/backend/imap/imap.php:1132

and

[FATAL] [<user>] Fatal error: /<path>/z-push/backend/imap/imap.php:1177 - Uncaught TypeError: in_array(): Argument #2 ($haystack) must be of type array, bool given in /<path>/z-push/backend/imap/imap.php:1177

Does this close any currently open issues?

Not that I know of.

Any relevant logs, error output, etc?

See above.

Where has this been tested?

Server (please complete the following information):

• OS: Debian 11
• PHP Version: 8.2
• Backend for: IMAP
• and Version: 7.1

Smartphone (please complete the following information):

• Device: Android
• OS: Android 13
• Mail App: Outlook
• Version: 4.2338.0

[matidau]

Thanks for this PR.

Will test and merge.

[dborg89]

Indeed this works!
Apologies for not noticing it in the commit history (just downloaded the latest stable from the releases section when I last updated).

Thanks :)