Werk #15941: Fix possible failed notifications on appliances

Component	Notifications
Title	Fix possible failed notifications on appliances
Date	Aug 21, 2023
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	2.3.0b1 Checkmk Community, Checkmk Pro, Checkmk Ultimate, Checkmk Ultimate MT 2.2.0p9 Checkmk Community, Checkmk Pro, Checkmk Ultimate, Checkmk Ultimate MT 2.1.0p33 Checkmk Community, Checkmk Pro, Checkmk Ultimate MT

If one of multiple recipients had no Email address configured, sending of the mail failed with an unhandled exception.

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