add agent to model.agent correctly even if super was not called

fix for the second issue in projectmesa#2006. If super is not present, we create the data structure but forget to add the agent to it. This is just a backward compatibility fix.

mesa/agent.py

@@ -54,6 +54,7 @@ def __init__(self, unique_id: int, model: Model) -> None:
         except AttributeError:
             # model super has not been called
             self.model.agents_ = defaultdict(dict)
+            self.model.agents_[type(self)][self] = None
             self.model.agentset_experimental_warning_given = False
 
             warnings.warn(