MBS-6426: Accept Edit::Release::Edit edits that delete release events

When accepting the edit, if we have a release event of the form
"<country: null, date: null>" simply omit inserting this release event.
Release events are inserted as a full set replacement, so this acts
like deletion.

This is needed to accept some old pre-schema-17 edits.
Merge remote-tracking branch 'ocharles-bitbucket/mbs-6426' into beta

* ocharles-bitbucket/mbs-6426:
  MBS-6426: Accept Edit::Release::Edit edits that delete release events