Is mandatory to play at least one skill card in skill checks?
The base rules, page 17, has an "Example of a Skill Check" (emphasis added):


  
  The player to the current player’s left plays two Skill Cards into the pile. The next player decides not to play any cards, and finally, the current player plays a single card.
  


I don't think they would have included this if a person had to play a skill card.