Include plate acquisition start time even if end time is missing

[melissalinkert]

This adds an extra case to the logic for setting the plate acquisition
name in the tree in the left-hand panel. If a start time (but not an
end time) was set, then the name will be the start time instead of Run $id.

To test, import plate-acquisition-test.ome.xml, which is a simple single-well plate with no EndTime on the PlateAcquisition. Verify that the starttime on plateacquisition is set to a valid timestamp in the database. Open the same plate in OMERO.web without this change, and verify that the corresponding plate acquisition is listed in the left-hand panel tree as Run followed by the ID number. With this change, open the plate again (no need to reimport) and verify that the plate acquisition is now listed as a timestamp matching the value of starttime.

[will-moore]

This seems to be working fine (once I'd worked out that the ome.xml sample above has the Acquisition name set to PlateAcquisition Name 0, so the timestamps weren't being used).

Before this PR:

With this PR: