Handles null values in MessageType and ContentType metadata fields when publishing external integration events.

Fixes #423
 Fix MessageFailedPublisher failing on deleted  FailedMessage

In some scenarios the ExpiredDocumentCleaner process could remove a
FailedMessage prior to it being dispatched.
Do not remove FailedMessage documents
Merge branch 'hotfix-1.4.2'
WIP
Merge branch 'master' into Issue-333
reworked installer to fix upgrading with mandatory ForwardAuditMessages