fix: Use model.schedule only when it is not None

[rht]

This fixes the error that shows up in projectmesa/mesa-examples#79 and projectmesa/mesa-examples#99,

        agent_records = map(
            get_reports,
>           model.schedule.agents if hasattr(model, "schedule") else model.agents,
        )
E       AttributeError: 'NoneType' object has no attribute 'agents'

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
Schelling	small	🔵 +0.2% [-0.3%, +0.6%]	🔵 -0.8% [-0.9%, -0.6%]
Schelling	large	🔵 +12.0% [-18.3%, +54.7%]	🔵 -2.3% [-4.3%, -0.2%]
WolfSheep	small	🔵 +1.2% [+0.6%, +1.8%]	🔵 -0.7% [-0.9%, -0.5%]
WolfSheep	large	🔵 +16.8% [-0.1%, +40.7%]	🔵 +0.1% [-1.2%, +1.8%]
BoidFlockers	small	🔵 +0.4% [-0.4%, +1.0%]	🔵 +0.2% [-0.5%, +1.0%]
BoidFlockers	large	🔵 -0.1% [-0.8%, +0.5%]	🔵 -0.8% [-1.2%, -0.3%]

[EwoutH]

Thanks, looks good!