Werk #12084: wrong command line argument usage in check_mail active check

Some command line arguments for the check_mail active check had been assembled in a way that allowed invalid values for "forward_app" or "match_subject". Activating "Forward mails as events to Event Console" in WATO without specifying forward app or a regex for subject would provide the sting "None" to forward-app and leave "match_subject" empty resulting in an Unhandled exception:

TypeError('first argument must be string or compiled pattern')

This change fixes command line argument generation and checks for "match_subject" being set.

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