OME-TIFF: return blank planes if the corresponding file does not exist

Fixes http://trac.openmicroscopy.org.uk/ome/ticket/12447.

Without this change, unzipping the datasets attached to the ticket, removing one .ome.tif file from each and importing should result in the missing file's well containing non-zero pixel data (as in the screenshot attached to the ticket).  The same test with this change should result in the missing file's well containing all-zero pixel data.

The "inversion" mentioned in the ticket is a result of the pixel type being set to int8 in the OME-XML stored in the original OME-TIFF files.  Setting the pixel type to uint8 in the OME-XML would solve this, but it's not a reader issue and so is not addressed here.