[wpilibj] Trajectory: Add zero-arg constructor (#2513)

C++ already has this.

wpilibj/src/main/java/edu/wpi/first/wpilibj/trajectory/Trajectory.java

@@ -26,6 +26,14 @@ public class Trajectory {
   private final double m_totalTimeSeconds;
   private final List<State> m_states;
 
+  /**
+   * Constructs an empty trajectory.
+   */
+  public Trajectory() {
+    m_states = new ArrayList<>();
+    m_totalTimeSeconds = 0.0;
+  }
+
   /**
    * Constructs a trajectory from a vector of states.
    *