Sometimes, all relays have weight=0

We've encountered an issue in some shadow simulations where all usable relays apparently have weight=0. With rand 0.8, this would make them get chosen uniformly; with rand 0.9, this makes them get rejected.

We're adding code (in !2880 (merged)) to temporarily emulate the behavior of rand 0.8 here, but it would be more logical to remove it. But:

• First, we want to make sure of why all the relays have weight=0 in this case.
• This may break CI ("this bug is load-bearing")

cc @jnewsome