The BlobsStitcher object will reassemble blobs which have been split up
across multiple gRPC messages. It is only used in the context of the
`get_blobs()` RPC right now, which has a bunch of information about
blobs which may not be available via other RPCs. While the stitcher
gracefully handles absence for many of the parameters, absence of a blob
mode and path will cause failure.

Relaxe the stitcher to work alright in case either mode or path are
absent. This is in preparation for the introduction of the
`list_blobs()` RPC.