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.

To test, verify that bfconvert succeeds on a dataset that does not have an acquisition date.  The logs from ```BIOFORMATS-5.1-merge-full-repository``` should also be cleaner; see http://ci.openmicroscopy.org/view/Bio-Formats/job/BIOFORMATS-5.1-merge-full-repository/214/console for comparison.