Merge pull request #4716 from Unity-Technologies/develop-fix-validate-action

[Fix] Validate Action

ml-agents-envs/mlagents_envs/base_env.py

             _size = self.continuous_size
         else:
             _size = self.discrete_size
-        _expected_shape = (n_agents, _size) if n_agents else (_size,)
+        _expected_shape = (n_agents, _size) if n_agents is not None else (_size,)
         if actions.shape != _expected_shape:
             raise UnityActionException(
                 f"The behavior {name} needs an input of dimension "