11472 empty min max string

See https://trac.openmicroscopy.org.uk/ome/ticket/11472

A script which has an empty max string should show as much from the CLI. Steps to test:

**1.** Create the following script and upload it as an official script
```
import omero
import omero.scripts as _os
import omero.rtypes as _ot
c = _os.client("test", _os.String("a", max=_ot.rstring(""), optional=False))
```
**2.** List the params for that script:
```id:  14798
name:  test
...
inputs:
  a - (no description)
    Optional: False
    Type: ::omero::RString
    Min: 
    Max: ''  <--- This is the important bit
    Values: 
outputs:
```