Mission feasibility checker: Handle mission with zero elements silently

A mission with zero elements is invalid but should not lead to a warning. Previously a fixed wing aircraft without a mission did emit warnings about a missing land item.

src/modules/navigator/mission_feasibility_checker.cpp

@@ -58,6 +58,11 @@ MissionFeasibilityChecker::checkMissionFeasible(const mission_s &mission,
 		float max_distance_to_1st_waypoint, float max_distance_between_waypoints,
 		bool land_start_req)
 {
+	// trivial case: A mission with length zero cannot be valid
+	if ((int)mission.count <= 0) {
+		return false;
+	}
+
 	bool failed = false;
 	bool warned = false;