full nodes can continue processing blocks if NextSequencersHash is empty

[srene]

UpdateProposerFromBlock sets dymint state proposer to nil if block.Header.NextSequencersHash is empty.
This can cause the next block to panic on validation because state.GetProposerHash() returns empty with following message
panic: runtime error: cannot convert slice with length 0 to array or pointer to array with length 32
imo full nodes should restart as well when next sequencer hash is empty, although this is actually only a problem in 3d migration.
in sentinel rotation (only case where next sequencer can be empty) a fork is created and fullnode is frozen when fork is detected anyway...