Are players required to make conquests on their turn?
No, it is not required. There's nothing wrong with just leaving everything exactly as is. 


  Leaving in place one Race token in each Region they occupy,
  the player may take all his other Active Race tokens from the
  map back in hand and use them to conquer new Regions. 


Note the "may".

Also:


  The active player can repeat this process to conquer as
  many new Regions as he wishes during his turn, provided he
  has enough Race tokens left to accomplish these successive
  conquests.


"As many new Regions as he wishes." 0 might be as many as you wish.