Change figure script logging for null dates.

Due to changes in https://github.com/openmicroscopy/openmicroscopy/pull/2847 the acquisition date can now be null. This caused these tests: http://ci.openmicroscopy.org/view/Failing/job/OMERO-5.1-merge-integration-python/199/testReport/test.integration.test_figureExportScripts/TestFigureExportScripts/ to fail. The failure is due to the four scripts logging the acquisition date. This PR changes that logging rather than removing it.

It's probably enough of a test if these four integration tests pass.

/cc @will-moore @mtbc