Read size remotely if value is null (Fix #11743)

If a file has not yet been updated by the server,
then the `size` field could still be null. In that
case, we ask the service to tell us the size.

See:
 * http://lists.openmicroscopy.org.uk/pipermail/ome-users/2013-November/004083.html
 * https://github.com/openmicroscopy/openmicroscopy/blob/v.5.0.0-beta1/components/insight/SRC/org/openmicroscopy/shoola/env/data/OMEROGateway.java#L3766