Fix NoneType position error in Hotelling’s Law SpaceDrawer

[ShreyasN707]

Summary

This PR fixes a TypeError in the Hotelling’s Law example that caused the visualization to crash whenever an agent didn’t have a valid grid position.

Bug / Issue

In examples/hotelling_law/app.py, the SpaceDrawer component assumes that agent.pos is always a tuple like (x, y).
However, starting with Mesa 3.2, agents can temporarily have pos = None when they're not placed on the grid.

When the code tried to access pos[0], it resulted in:

TypeError: 'NoneType' object is not subscriptable

Fix

I updated SpaceDrawer to safely handle agents whose positions are None:

• Added a simple if agent.pos is None: continue in the loop that builds the cell contents for store agents.

• Added the same check in the loop that draws the scatter plot.

With these guards, agents without positions are skipped during rendering, preventing the crash.

Testing

• Manual testing: Confirmed that solara run app.py now runs without errors and the visualization loads correctly.

• Automated testing: Ran pytest test_examples.py locally. All 15 tests passed, so the underlying model behavior remains unchanged.

Additional Notes

This update only adjusts the visualization logic to match Mesa 3.2+ behavior regarding pos=None.
No changes were made to the simulation itself.