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 Raw (CRE), Checkmk Enterprise (CEE), Checkmk Cloud (CCE), Checkmk MSP (CME) 2.2.0p9 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk Cloud (CCE), Checkmk MSP (CME) 2.1.0p33 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

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

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