fix #10727, so raw file bean can create empty files

To test, in Insight try attaching a file of zero size to an image and see if it works and downloads okay. See notes on https://trac.openmicroscopy.org.uk/ome/ticket/10712 and https://trac.openmicroscopy.org.uk/ome/ticket/10727

Also check that `originalfile.sha1` in the database has the correct hash for an empty file, viz.,
```
$ shasum /dev/null
da39a3ee5e6b4b0d3255bfef95601890afd80709  /dev/null
```