Werk #12752: jira notification: Fix empty rule on edit if rule was created pre 1.6.0p18

With werk #11202 the option "Site custom field ID" was introduced. If you configured rules before that werk and tried to edit such rule in versions from 1.6.0p18 upwards, an empty rule was shown.

A workaround was, to edit the file /var/check_mk/conf.d/wato/notifications.mk and add the key value pair "'site_customid': ''" to the "jira_issues" notification rule.

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