Fix for non-existing Pixels directory in bin/omero admin cleanse

See https://www.openmicroscopy.org/community/viewtopic.php?f=5&t=7540 for a description of the exception

To test this PR, run
```
bin/omero admin cleanse --dry-run
```
1. against a fresh 5.0.x server (no `Pixels` directory) and check the `Pixels` directory is skipped
2. against a 5.0.x server with a 4.4 DB upgrade, e.g. octopus.openmicroscopy.org and check the `Pixels` directory is still scanned.
should not fail with an exception
It's probably safe to also skip the other directories (`Files` and `Thumbnails`).