Remove usage of CHECK_NOTNULL

We want to switch to absl logging instead of glog.  gtest and ceres are
going there, and we already have absl as a dependency.  ABSL doesn't
have CHECK_NOTNULL, and we can move things over in an easier to review
fashion.

Change-Id: Ifd9a11ec34a2357cec43f88dba015db9c28ed2cf
Signed-off-by: Austin Schuh <austin.linux@gmail.com>
diff --git a/frc971/control_loops/catapult/catapult.cc b/frc971/control_loops/catapult/catapult.cc
index a827dc9..153d35c 100644
--- a/frc971/control_loops/catapult/catapult.cc
+++ b/frc971/control_loops/catapult/catapult.cc
@@ -64,7 +64,7 @@
         std::optional<double> solution = catapult_mpc_.Next();
 
         if (!solution.has_value()) {
-          CHECK_NOTNULL(catapult_voltage);
+          CHECK(catapult_voltage != nullptr);
           *catapult_voltage = 0.0;
           if (catapult_mpc_.started()) {
             ++shot_count_;
@@ -73,7 +73,7 @@
           }
         } else {
           // TODO(austin): Voltage error?
-          CHECK_NOTNULL(catapult_voltage);
+          CHECK(catapult_voltage != nullptr);
           if (current_horizon_ == 1) {
             battery_voltage = 12.0;
           }