Werk #15914: active_checks_mailbox: Fix rule transform when updating from 2.1.0p25 or lower

If the rule for any mailbox related active check was configured without defining the 'server' option and an update to a higher version was attempted, it caused a rule transform error. This caused the update to fail. The error looks similar to this:

client_id, client_secret, tenant_id = auth_data
ValueError: not enough values to unpack (expected 3, got 2)

This has now been fixed and the transformation of the rule will not cause any problems while updating.

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