cell_space: Allow CellCollection to be empty

[EwoutH]

Sometimes a CellCollection needs to be empty, especially after filtering with .select(). However, the code would raise a StopIteration error trying to get capacity from the first cell. This PR handles empty collections gracefully.

Summary

Fix StopIteration error when creating empty CellCollections by allowing capacity to be None when there are no cells.

Bug / Issue

Creating a CellCollection with no cells (after filtering) raises StopIteration:

# This raises StopIteration if no cells meet the condition
cells = neighbors.select(lambda cell: not any(isinstance(obj, Wolf) for obj in cell.agents))

Implementation

Check for empty collection before accessing first cell's capacity:

self._capacity: int | None = (
    next(iter(self._cells.keys())).capacity if self._cells else None
)

Testing

• Verify empty CellCollection creation
• Test select() with no matching cells
• Check behavior of empty collection methods

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔵 -0.6% [-1.0%, -0.1%]	🔵 -0.3% [-0.4%, -0.1%]
BoltzmannWealth	large	🔵 -0.9% [-1.2%, -0.5%]	🔵 -1.0% [-1.2%, -0.8%]
Schelling	small	🔵 -1.4% [-1.7%, -1.1%]	🔵 +0.1% [-0.2%, +0.3%]
Schelling	large	🔵 -1.2% [-1.4%, -0.9%]	🔵 +0.5% [+0.2%, +1.0%]
WolfSheep	small	🔵 +0.0% [-0.2%, +0.3%]	🔵 +0.8% [+0.6%, +0.9%]
WolfSheep	large	🔵 +0.1% [-0.5%, +0.8%]	🔵 +2.5% [+1.6%, +3.8%]
BoidFlockers	small	🔵 -0.5% [-1.1%, +0.1%]	🔵 +0.3% [-0.6%, +1.3%]
BoidFlockers	large	🔵 -0.2% [-1.2%, +0.8%]	🔵 +0.6% [-0.2%, +1.3%]