Make fetching PDP values optional

This is important for y2012, which doesn't have one.

Change-Id: Ib0cfb846424b7a8acbf071047692064abb2a487c
diff --git a/frc971/wpilib/wpilib_interface.cc b/frc971/wpilib/wpilib_interface.cc
index 5df3c9c..8d67cf0 100644
--- a/frc971/wpilib/wpilib_interface.cc
+++ b/frc971/wpilib/wpilib_interface.cc
@@ -28,7 +28,9 @@
   new_state->voltage_roborio_in = ControllerPower::GetInputVoltage();
   new_state->voltage_battery = ds->GetBatteryVoltage();
 
-  pdp_fetcher->GetValues(&new_state->pdp);
+  if (pdp_fetcher) {
+    pdp_fetcher->GetValues(&new_state->pdp);
+  }
 
   LOG_STRUCT(DEBUG, "robot_state", *new_state);