Fix gazebo model direction

[takanotume24]

Solved the problem shown in #53 where Gazebo's 3D model is always facing the same direction, which was due to the fact that there was no value in AgentState's pose.orientation in the first place. I've modified the code by referring to this. https://github.com/srl-freiburg/pedsim_ros/blob/b21218347149657f0639ada23da8847f537811ce/pedsim_visualizer/src/sim_visualizer.cpp#L98

[makokal]

Thanks for this fix.