[wpilibj] DrivetrainSim: Initialize m_u to default value (#2854)

m_u wasn't being initialized, so if user called update() before setInputs() the program would crash with an NPE.

wpilibj/src/main/java/edu/wpi/first/wpilibj/simulation/DifferentialDrivetrainSim.java

@@ -98,6 +98,7 @@ public class DifferentialDrivetrainSim {
     m_currentGearing = m_originalGearing;
 
     m_x = new Matrix<>(Nat.N7(), Nat.N1());
+    m_u = VecBuilder.fill(0, 0);
   }
 
   /**