Werk #7339: Fixed broken mail notifications with Nagios core (1.5.0p14 regression)

Unfortunately, version 1.5.0p14 contained a bug that prevented mail notifications when using the Nagios core. According to our knowledge users of the Microcore are not affected.

The error is displayed as follows in var/log/notify.log:

2019-04-17 09:15:19 Preparing rule based notifications
2019-04-17 09:15:19 Found 0 user specific rules
2019-04-17 09:15:19 Global rule 'Notify all contacts of a host/service via HTML email'...
2019-04-17 09:15:19  -> matches!
2019-04-17 09:15:19    - adding notification of lm via mail
2019-04-17 09:15:19 Executing 1 notifications:
2019-04-17 09:15:19   * notifying lm via mail, parameters: host_subject, from, service_subject, disable_multiplexing, bulk: no
2019-04-17 09:15:19      executing /omd/sites/stable/share/check_mk/notifications/mail
2019-04-17 09:15:19      Output: Cannot send HTML email: empty destination email address
2019-04-17 09:15:19      Plugin exited with code 2

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