Incorrect handling of not found proof of work

### Describe the problem you discovered

When an incoming transaction has an invalid origin signature, the coordinator node does not retrieve the proof of work.
The proof of work returns an empty string. Then the validation stamp is serialized replacing the empty string with `<<0::32>>`.
Then the cross validators compare the proof of work they found (empty string) with the one in the validation stamp (`<<0::32>>`) which does not match while all nodes did not found the POW.
This result into a wrong consensus not reach error

### Describe the solution you'd like

Define a better workflow for invalid proof of work

### Epic

_No response_