fix mock brain prev action

ml-agents/mlagents/trainers/tests/mock_brain.py

             if action_spec.is_discrete()
             else None
         )
-        prev_action = None
+        else:
+            prev_action = np.ones(action_size, dtype=np.float32)
 
         max_step = False
         memory = np.ones(memory_size, dtype=np.float32)