Switch `spssStake` to use `NonZero`

[lehins]

After we are in PV11 we can get rid of stake pools with zero stake from the SnapShot

However, this is blocked #5014, since PoolDistr is currently reused for calculation of the stake for the voting stake, which means that stake pools that don't have any stake for leader election might still be needed for stake for voting if any of the stake credential without stake is also associated with a governance proposal that holds a deposit.