run_dqn demo fails with distributed training: ValueError: TrajectoryColumns cannot contain any None data references

Ubuntu 20.04
cuda-11.4
4 GPU / 4 CPU core node

Setup (from fresh VM):
```shell
> apt-get update && apt-get install -y --no-install-recommends \
libgl1-mesa-glx libosmesa6 libglew-dev
> pip install --upgrade pip setuptools wheel
> git clone https://github.com/deepmind/acme.git acme_repo
> cd acme_repo
> pip install .[jax,tf,testing,envs]
> pip install --upgrade "jax[cuda]" -f https://storage.googleapis.com/jax-releases/jax_releases.html
> pip install ale-py
```

Your favorite method to install atari rooms into ale-py here

Then:

```shell
> cd examples/baselines/rl_discrete
> python run_dqn.py --run_distributed
```

Produces this error:
```
I0525 17:21:19.286271 139675943556864 terminal.py:91] [Actor] Actor Episodes = 1 | Actor Steps = 186 | Episode Length = 186 | Episode Return = -5.0 | Steps Per Second = 1.972
Node ThreadWorker(thread=, future=) crashed:
Traceback (most recent call last):
File "/usr/local/lib/python3.8/dist-packages/launchpad/launch/worker_manager.py", line 474, in _check_workers
worker.future.result()
File "/usr/lib/python3.8/concurrent/futures/_base.py", line 437, in result
return self.__get_result()
File "/usr/lib/python3.8/concurrent/futures/_base.py", line 389, in __get_result
raise self._exception
File "/usr/local/lib/python3.8/dist-packages/launchpad/launch/worker_manager.py", line 250, in run_inner
future.set_result(f())
File "/usr/local/lib/python3.8/dist-packages/launchpad/nodes/python/node.py", line 75, in _construct_function
return functools.partial(self._function, *args, **kwargs)()
File "/usr/local/lib/python3.8/dist-packages/launchpad/nodes/courier/node.py", line 130, in run
instance.run()
File "/usr/local/lib/python3.8/dist-packages/acme/environment_loop.py", line 176, in run
result = self.run_episode()
File "/usr/local/lib/python3.8/dist-packages/acme/environment_loop.py", line 109, in run_episode
self._actor.observe(action, next_timestep=timestep)
File "/usr/local/lib/python3.8/dist-packages/acme/agents/jax/actors.py", line 94, in observe
self._adder.add(
File "/usr/local/lib/python3.8/dist-packages/acme/adders/reverb/transition.py", line 133, in add
super().add(*args, **kwargs)
File "/usr/local/lib/python3.8/dist-packages/acme/adders/reverb/base.py", line 207, in add
self._write()
File "/usr/local/lib/python3.8/dist-packages/acme/adders/reverb/transition.py", line 169, in _write
reward, discount = tree.map_structure(
I0525 17:21:20.443459 139678827484928 lp_utils.py:95] StepsLimiter: Reached 186 recorded steps
File "/usr/local/lib/python3.8/dist-packages/tree/__init__.py", line 430, in map_structure
[func(*args) for args in zip(*map(flatten, structures))])
File "/usr/local/lib/python3.8/dist-packages/tree/__init__.py", line 430, in
[func(*args) for args in zip(*map(flatten, structures))])
File "/usr/local/lib/python3.8/dist-packages/acme/adders/reverb/transition.py", line 151, in
get_all_np = lambda x: x[self._first_idx:self._last_idx].numpy()
File "/usr/local/lib/python3.8/dist-packages/reverb/trajectory_writer.py", line 604, in __getitem__
return TrajectoryColumn(self._slice(val), path=path)
File "/usr/local/lib/python3.8/dist-packages/reverb/trajectory_writer.py", line 629, in __init__
raise ValueError('TrajectoryColumns cannot contain any None data '
ValueError: TrajectoryColumns cannot contain any None data references: TrajectoryColumn at path ('reward', slice(187, 188, None)) got [None].
```

Without the distributed flag, it works fine.