Use `isEmpty` to improve performance in Pregel's superstep

object Pregel extends Logging {
  ...
  def apply[VD: ClassTag, ED: ClassTag, A: ClassTag] (...): Graph[VD, ED] = {
    ...
    // Maybe messages.isEmpty() is better than messages.count()
    var activeMessages = messages.count()

    // Loop
    var prevG: Graph[VD, ED] = null
    var i = 0
    while (activeMessages > 0 && i < maxIterations) {
      ...
      activeMessages = messages.count()
      ...
    }
    ...
    g
  } // end of apply

} // end of class Pregel