Require non null (#580)

* Replace null checks with Objects.requireNonNull()

* Use PMD rule instead of checkstyle rule

wpilibjIntegrationTests/src/main/java/edu/wpi/first/wpilibj/fixtures/MotorEncoderFixture.java

@@ -170,6 +170,7 @@ public abstract class MotorEncoderFixture<T extends SpeedController> implements
    * deallocated.
    */
   @Override
+  @SuppressWarnings("Regexp")
   public boolean teardown() {
     String type;
     if (m_motor != null) {