AP_L1_Control: Use is_positive

Avoid !is_zero

libraries/AP_L1_Control/AP_L1_Control.cpp

@@ -553,7 +553,7 @@ void AP_L1_Control::update_level_flight(void)
 // update L1 control for path following
 void AP_L1_Control::follow_path(const Location &location_on_path, const Vector2f& unit_path_tangent, float path_curvature, int8_t direction) {
     //! @note initial implementation uses existing functions
-    if (!is_zero(path_curvature)) {
+    if (is_positive(path_curvature)) {
         // moving along arc of circle - loiter about wp located at
         // centre of curvature.
         float radius_m = 1.0 / path_curvature;