do not insert acquisition dates by default unless from metadata

https://github.com/openmicroscopy/openmicroscopy/pull/2847 enables `null` acquisition dates and this PR is the remaining piece for http://trac.openmicroscopy.org.uk/ome/ticket/12465.
--no-rebase

Testing: with https://github.com/openmicroscopy/openmicroscopy/pull/2847 also merged, importing an image format like PNG that does not have acquisition dates, should create a resulting `Image` that has a `null` acquisition date.