fix default random action

ml-agents-envs/mlagents_envs/base_env.py

         continuous = np.random.uniform(
             low=-1.0, high=1.0, size=(n_agents, self.continuous_size)
         )
-        discrete = np.array([])
+        discrete = np.zeros((n_agents, self.discrete_size), dtype=np.int32)
         if self.discrete_size > 0:
             discrete = np.column_stack(
                 [