Don't use rstring on null values (fix #12056).

This PR fixes image description preservation between exporting/importing to the server. To test, verify that setting an image description on an image doesn't get lost when that image is exported as OME-TIFF and the re-imported to the server (see https://trac.openmicroscopy.org.uk/ome/ticket/12056).

\cc @joshmoore: Is that the best place the null check can be placed? Or can it be done even earlier in the code?