Add ways to remove 0-byte pyramid files

See https://trac.openmicroscopy.org.uk/ome/ticket/11395

There are two approaches in this PR: one is to automatically remove 0-byte pyramid files automatically when a ```BfPixelBuffer``` attempts to load them, and the other is the ```bin/omero admin fixpyramids``` command which will remove all 0-byte pyramid files that do not have a lock or temp file.

```bin/omero admin fixpyramids``` should satisfy the ticket requirements, but I'd be happy to move that logic to a different one of the CLI plugins and/or change ```fixpyramids``` to something else.

I'm not 100% sure of the ```BfPixelBuffer``` changes - the impact shouldn't be huge, but I don't know to what extent that will cause an infinite pyramid generation loop with seriously broken data.  If it's going too far, happy to revert the corresponding commit.