Skip megapools with no active validators when calculating ETH Rewards

[thomaspanf]

This PR resolves the following:

rocketpool_watchtower  | 2026/02/15 22:01:11 [Balances] Approximating tree using Ruleset v11.
rocketpool_watchtower  | 2026/02/15 22:01:11 [Balances] Creating tree for 161 nodes
rocketpool_watchtower  | 2026/02/15 22:01:11 [Balances] Smoothing Pool Balance:    76911844449941980    (0.077)
rocketpool_watchtower  | 2026/02/15 22:01:11 [Balances] Voter Share from Megapools not in the smoothing pool:    764172624442500000    (0.764)
rocketpool_watchtower  | 2026/02/15 22:01:11 [Balances] Getting details of nodes for Smoothing Pool calculation...
rocketpool_watchtower  | 2026/02/15 22:01:11 [Balances] 127 / 161 nodes (30 with megapools) were eligible for Smoothing Pool rewards
rocketpool_watchtower  | panic: division by zero
rocketpool_watchtower  | 
rocketpool_watchtower  | goroutine 9129 [running]:
rocketpool_watchtower  | math/big.nat.div({0x0?, 0x4000000000006?, 0x7eff0c745158?}, 0x7eff6d7a0108?, {0x0?, 0xc000101808?, 0xc0028f2260?}, {0xc001834900, 0x0, 0x8}, ...)
rocketpool_watchtower  |     /usr/local/go/src/math/big/natdiv.go:520 +0x4b5
rocketpool_watchtower  | math/big.(*Int).QuoRem(0xc0028f2260, 0xc00181dd40, 0xc00004b410, 0xc00004b3c0)
rocketpool_watchtower  |     /usr/local/go/src/math/big/int.go:300 +0x8c
rocketpool_watchtower  | math/big.(*Int).Div(0xc0028f2260, 0x6?, 0x2?)
rocketpool_watchtower  |     /usr/local/go/src/math/big/int.go:311 +0x38
rocketpool_watchtower  | github.com/rocket-pool/smartnode/bindings/utils/state.(*NativeMegapoolDetails).GetMegapoolBondNormalized(0xc00004b8f0)
rocketpool_watchtower  |     /src/bindings/utils/state/megapool.go:47 +0x8d
rocketpool_watchtower  | github.com/rocket-pool/smartnode/shared/services/rewards.(*treeGeneratorImpl_v11).calculateEthRewards(0xc000570960, 0x0)
rocketpool_watchtower  |     /src/shared/services/rewards/generator-impl-v11.go:585 +0x26c5

Also includes a minor optimization by setting the network level values nodeFee, voterFee, pdaoFee and megapool bond once per megapool instead of once per validator.