Don't pass invalid ICE_CONFIG to Ice (fix #10051).

This PR fixes https://trac.openmicroscopy.org/ome/ticket/10051.

To test, follow the steps from the ticket, with a tiny modification:
- Start the server
- Login in via `bin/omero login`
- Set the `ICE_CONFIG` env var (```export ICE_CONFIG=`bin/omero sessions file` ```)
- Log out (`bin/omero logout`)
- Stop the server
- Try to start the server
- Verify that no error message appears

This should also work when ICE_CONFIG is set to any invalid file/directory.