Support diff generation with null OIDs in `DiffBlobs`

The DiffBlobs RPC generates and streams diffs for request blob pairs. Unfortunately git-diff(1) does not understand null OIDs though. This means that for additions and deletions the DiffBlobs RPC cannot effectively generate diffs. It could be argued that one should not request a diff for a created/deleted file because it would simply be the contents of the file prepended with +/- respectively. At the same time, from an API design and convenience perspective, it would be simpler and more intuitive to have Gitaly handle these cases on behalf of the callers.