Fixes issue with undefined action and probs in qlearner.
Fixes issue with undefined action and probs in qlearner.

Specifically, in some sequential games, certain players may have never taken an action when the game terminates. Then, _prev_info_state would be None, while action & probs would never have been defined, raising an error when setting "self._prev_action = action" in the qlearner code.

PiperOrigin-RevId: 293833451
Change-Id: I0b95fb8cc192e1a700e11f97faa4a830297f3043

deepmind/open_spiel,deepmind/open_spiel,deepmind/open_spiel,deepmind/open_spiel,deepmind/open_spiel,deepmind/open_spiel