ProcessSession::read can't read zero length flowfiles (DatabaseContentRepository)

RocksDbStream::write will not create a key in the db for data with length 0, but RocksDbStream::read will check for the existence of the key even when the length is 0.

We could either

•  fix RocksDbStream::write to create an entry in the db even for zero length values or
•  fix RocksDbStream::read to return successfully when reading 0 bytes even if there is no corresponding key