Fix crash in construct_curr_info when next_info doesn't have any agents (#2549)

Fixes #1687

ml-agents/mlagents/trainers/rl_trainer.py

                 agent_brain_info.previous_text_actions[agent_index]
             )
             action_masks.append(agent_brain_info.action_masks[agent_index])
-        if self.policy.use_recurrent:
+        # Check if memories exists (i.e. next_info is not empty) before attempting vstack
+        if self.policy.use_recurrent and memories:
             memories = np.vstack(memories)
         curr_info = BrainInfo(
             visual_observations,