Apologies if this has been discussed before, but it's one of the last remaining questions I have left after watching a lot of walkthroughs and reading a lot of posts. I'm playing solo and have cleared the staging area at the end of a round. On the next round I commit characters to a quest and the encounter card I then reveal has no threat level (i.e. a treachery card). Does that phase become void or is the enemy threat then zero?

The threat in the staging area is 0, and you would resolve the quest phase normally, comparing that number to your committed willpower.

Meaning you would make progress on the active location and quest equal to the amount of willpower you committed, since you revealed no threat to reduce it.