Resolve unsoundness caught by pytype --strict-none-binding.

PiperOrigin-RevId: 882611409
Change-Id: If184574edd2ff3bb4f16bb6500ba543bd7ba8525

Author: Acme Contributor

acme/agents/jax/pwil/adder.py

@@ -38,6 +38,7 @@ def add(self,
           action: types.NestedArray,
           next_timestep: dm_env.TimeStep,
           extras: types.NestedArray = ()):
+    assert self._latest_observation is not None
     updated_timestep = next_timestep._replace(
         reward=self._rewarder.append_and_compute_reward(
             observation=self._latest_observation, action=action))