Deltavision: do not warn if the ND is set to "BLANK"

Noticed by @joshmoore.

This should prevent things like:

```
2013-10-28 10:07:51,499 WARN  [              loci.formats.FormatHandler] (l.Server-2) Could not parse ND filter 'BLANK'
2013-10-28 10:07:51,499 WARN  [              loci.formats.FormatHandler] (l.Server-2) Could not parse ND filter 'BLANK'
2013-10-28 10:07:51,499 WARN  [              loci.formats.FormatHandler] (l.Server-2) Could not parse ND filter 'BLANK'
2013-10-28 10:07:51,499 WARN  [              loci.formats.FormatHandler] (l.Server-2) Could not parse ND filter 'BLANK'
2013-10-28 10:07:51,500 WARN  [              loci.formats.FormatHandler] (l.Server-2) Could not parse ND filter 'BLANK'
2013-10-28 10:07:51,500 WARN  [              loci.formats.FormatHandler] (l.Server-2) Could not parse ND filter 'BLANK'
2013-10-28 10:07:51,500 DEBUG [                   loci.formats.Memoizer] (l.Server-2) skipping save memo. elapsed millis: 25
2013-10-28 10:07:51,500 DEBUG [                   loci.formats.Memoizer] (l.Server-2) start[1382954871475] time[25] tag[loci.formats.Memoizer.setId]
2013-10-28 10:07:51,500 INFO  [                ome.io.nio.PixelsService] (l.Server-2) Creating BfPixelBuffer:
/OMERO/ManagedRepository/user-1_8/2013-09/18/10-05-41.286/IAGFP-Noc01_R3D.dv Series: 0
```