Fix empty string error

This PR fixes issue noticed by Sebastian Rhode and reported to ome-users at http://lists.openmicroscopy.org.uk/pipermail/ome-users/2014-August/004675.html

Test by importing the file:
https://www.dropbox.com/s/erjeucsabgzm1i2/EB3_H2B_z%20over%20time.czi