examples: Fix boid_flockers viz

[EwoutH]

Boids didn't have neighbours anymore (I broke it in #2473, sorry!), so the visualisation crashes. This commit fixes that.

[github-actions]

Performance benchmarks:

Model	Size	Init time [95% CI]	Run time [95% CI]
BoltzmannWealth	small	🔵 -0.3% [-1.5%, +0.9%]	🔵 +0.4% [+0.2%, +0.6%]
BoltzmannWealth	large	🔵 +0.6% [+0.0%, +1.3%]	🔵 +1.6% [+0.7%, +2.5%]
Schelling	small	🔵 +0.9% [+0.6%, +1.1%]	🔵 +0.1% [-0.1%, +0.3%]
Schelling	large	🔵 +1.1% [+0.6%, +1.6%]	🔵 +0.1% [-1.0%, +1.1%]
WolfSheep	small	🔵 +1.3% [+1.0%, +1.7%]	🔵 +0.2% [+0.1%, +0.4%]
WolfSheep	large	🔵 +1.6% [+1.2%, +2.1%]	🔵 -0.1% [-1.2%, +1.0%]
BoidFlockers	small	🔵 -0.2% [-0.7%, +0.2%]	🔵 -1.0% [-1.9%, -0.2%]
BoidFlockers	large	🔵 +0.8% [-0.1%, +1.7%]	🔵 -0.2% [-0.6%, +0.3%]