Make hql output more consistent

This is an attempt to fix the issue raised in http://trac.openmicroscopy.org.uk/ome/ticket/12500

As elements in a new row are populated by `None` by default these elements can be change to empty strings as in the methods `col()`. The alternative would be to backfill with `None` in `col()` though empty elements look neater.

To test do something like this:
```
bin/omero obj new Dataset name=foo
bin/omero obj new Dataset name=bar description="A description"
bin/omero obj new Dataset name=foo2
bin/omero hql "from Dataset"
```
The output with this fix should be:
```
 # | Class    | Id | name | details         | description   
---+----------+----+------+-----------------+---------------
 0 | DatasetI | 1  | foo  | owner=2;group=3 |               
 1 | DatasetI | 2  | bar  | owner=2;group=3 | A description 
 2 | DatasetI | 3  | foo2 | owner=2;group=3 |               
```
Prior to this PR the final element of row 2 would have been `None`.

/cc @joshmoore @mtbc