fix NoneType when depends are not specified (fix #245)
fix NoneType when depends are not specified (fix #245)

sorki/rosdep,allenh1/rosdep,alessandro-aglietti/rosdep,sorki/rosdep,aymanim/rosdep,alessandro-aglietti/rosdep,wkentaro/rosdep,georgepar/rosdep,spaghetti-/rosdep,ros-infrastructure/rosdep,aymanim/rosdep,allenh1/rosdep,spaghetti-/rosdep,ros-infrastructure/rosdep,georgepar/rosdep,wkentaro/rosdep