Right now there is zero incentive for Criminals to finish objectives when assigned a mission without opposition, This is clearly a waste of the Witness system which was clearly intended to be a core game mechanic. Therefore Criminals should get normal money and standing for completing objectives/missions even without opposition while Enforces get extra money and standing for witnessing a Criminal in a mission without opposition.
If Criminals actually completed objectives in missions without opposition it would give Enforcers a chance to witness the Criminals and start a mission, If Enforcers got extra money and standing for witnessing Criminals in a mission then Enforces would have a reason to search for Criminals to witness. It's very boring as a Criminal to get missions without opposition because you sit there on an 99% finish objective just waiting for opposition to show up, If missions without opposition gave normal money and standing then it suddenly becomes a fun and worthwhile challenge to try and finish a mission without being witnessed by Enforcers.
Criminals being able to farm not just money but also standing on empty servers isn't an issue because getting a mission without opposition requires readying up for a mission which places you on the matchmaker which continues to search for opposition, Which means that as soon as an Enforcer joins the server and readies up for a mission you could get opposition instantly which doesn't happen with ramraiding which already exists. There is also "A civilian has witnessed you" which I don't think does anything right now but what it could do is give any Enforcer who enters your mission from the matchmaker the same money and standing bonus they would have gotten if they had witnessed you.
Honestly this all seems to be how the game was supposed to work on release but they simply forgot to turn on rewards for missions without opposition making the witness system pointless.