AP_Math: allow null pointers in Quaternion::to_euler()

this matches the Matrix3f method

libraries/AP_Math/quaternion.cpp

@@ -76,10 +76,16 @@ void Quaternion::from_euler(float roll, float pitch, float yaw)
 // create eulers from a quaternion
 void Quaternion::to_euler(float *roll, float *pitch, float *yaw)
 {
+    if (roll) {
         *roll = -(atan2(2.0*(q1*q2 + q3*q4),
                         1 - 2.0*(q2*q2 + q3*q3)));
+    }
+    if (pitch) {
         // we let safe_asin() handle the singularities near 90/-90 in pitch
         *pitch = -safe_asin(2.0*(q1*q3 - q4*q2));
+    }
+    if (yaw) {
         *yaw = atan2(2.0*(q1*q4 + q2*q3),
                      1 - 2.0*(q3*q3 + q4*q4));
+    }
 }