BUG Saving page from another tab than content causes !isset error

When a workflow is in effect and WorkflowEmbargoExpiryExtension is active the
code wrongly assumes that the DesiredPublishDate and DesiredPublishDate is
always set.
Merge pull request #184 from stojg/pull/guard-against-saving-from-other-tab

BUG Saving page from another tab than content causes !isset error