cell_space: Allow CellCollection to be empty

Sometimes it's useful to have any empty cell collection in the cell space, which can happen after selection with no cells that meet the selection requirements.

However, self._capacity would give an error, because there were no cell to derive the capacity from. This PR resolves that error.

mesa/experimental/cell_space/cell_collection.py

@@ -48,8 +48,10 @@ def __init__(
         else:
             self._cells = {cell: cell.agents for cell in cells}
 
-        #
-        self._capacity: int = next(iter(self._cells.keys())).capacity
+        # Get capacity from first cell if collection is not empty
+        self._capacity: int | None = (
+            next(iter(self._cells.keys())).capacity if self._cells else None
+        )
 
         if random is None:
             warnings.warn(