Using Dynamic Content, the solution didn't go well. The Compose step worked (even though the expression didn't change to the pink fx box), however, although the Condition left-side let me enter Dynamic Content, it said the expression was INVALID.
empty(body('Search_for_users_(V2)_body('Request_Item_Created_in_SharePoint_List')?['Chair']')?['value']
I entered the Dynamic Content (Chair) i the same spot as Carlos in your example. I assume this expression would work, but it is missing some symbols?
PS- I tried various other things based upon your solutions, will no success. I especially tried right-side conditions of expressions of True, null, 0 and empty (empty gave invalid expression).
What is frustrating is that when a Search for user yields no results, it is listed as Outcomes, Value, []. To me this seems like outcomes equals nothing (or empty). Seems like the empty expression on the right-side should work, but instead, it is listed as invalid.
After not being able to enter your expression, I tried Outputs.

Also please notice that the Compose yields what was entered in the Chair field, not sure this will help divert to a Yes/NO path.

Results.
