Ensure nils are removed from sequences

[SteveNewson]

When a sequence is used, nils trigger a go routine in tea.go here:

case sequenceMsg:
  go func() {
    // Execute commands one at a time, in order.
    for _, cmd := range msg {
      if cmd == nil {
        continue
      }
      //...
    }
  }()

Even though the go routine is short-lived, this can cause a significant performance hit to the runtime. The Batch mechanism eliminates nils before they are processed. This change replicates that behaviour in sequenceMsg.

[SteveNewson]

This issue hit me harder because I was using a sequence in an Update() method where a batch would have been more appropriate. Since the majority of updates resulted in no new commands, I was generating a lot of sequences with nils in them. This resulted in an extremely laggy UI.