Fix geodesic_reward==None bug

[hang-yin]

Original issue: https://github.com/StanfordVL/OmniGibson/issues/715

The problem here is our get_shortest_path function returns None when no path is found but this is directly fed into a numerical computation for rewards: (self._potential - new_potential)*self._r_potential.

The proposed fix here is to pass in a float no_path_reward arg. I'm not super familiar with this RL infra though, would appreciate input on where this no_path_reward should be exposed to the users.