Do not accept empty delta elements

Do not accept empty delta elements

Ties de Kock found RRDP content that doesn't match the XML schema, most
likely generated by krill: https://github.com/NLnetLabs/krill/issues/1180

Use the state machine to mark a new delta element as empty and check at
the end whether that state was changed (which means it contained publish
or withdraw elements). If so, raise a parse failure.

ok claudio job