Prevent an NPE when referencing Image.AcquisitionDate

The acquisition date is now optional, and so may be null.  Without this
commit, bfconvert threw an NPE when the acquisition date was missing,
and the automated data tests logged a warning for each missing date.
Merge pull request #1264 from melissalinkert/date-npe

Prevent an NPE when referencing Image.AcquisitionDate