Two small blob fixes

Instead of swallowing all exceptions in Blob/Async#getSize() to cope with non-existent or invalid data, check for null to handle the former and let the latter propagate up. Hiding the error (incompatible data in the subspace)  ust punts it a little bit farther down the line, which doesn't seem very helpful.

In getChunkAt() the last key before the given offset is read. In general, this may be outside the blob subspace. Avoid attempting to interpret them as Tuples and just compare the raw bytes for the exit check.