ThreadNet support for pools with 0 stake in Praos tests

See Issue input-output-hk/ouroboros-network#2557 for an example of it happening in RealTPraos.

It can't yet happen in Cardano ThreadNet tests because those don't yet generate arbitrary Byron and Shelley transactions, so we won't see the requisite successful parameter update.

But we'll need to support that eventually, and we might need to address 0 stake for some other reason too. So this is the Issue to track support for it.

See Issue input-output-hk/ouroboros-network#2558 for a little more historical context.