PHP8 deprecation warning in MailhandlerPhpImapRetrieve

Problem/Motivation

On upgrade to PHP 8.1, this error appears in watchdog on cron:

Deprecated function: imap_open(): Passing null to parameter #4 ($flags) of type int is deprecated in MailhandlerPhpImapRetrieve->open_mailbox() (line 130 of /xxxx/xxxx/public_html/sites/all/modules/contrib/mailhandler/modules/mailhandler_php_imap/plugins/mailhandler/retrieve/MailhandlerPhpImapRetrieve.class.php).

Steps to reproduce

Upgrade to PHP 8.1 and watch log.

Proposed resolution

Patch coming.

Remaining tasks

Review, commit.

User interface changes

None

API changes

None

Data model changes

None