dApp staking v3 - claim staker benchmark fix

[Dinonard]

Pull Request Summary

Both a minor improvement & bug fix in claim staker rewards benchmark.

There were issues depending on whether the Build&Earn duration was longer or shorter than EraRewardSpanLength.

[github-actions]

Package	Line Rate	Branch Rate	Health
pallets/astar-xcm-benchmarks/src	89%	0%	✔
precompiles/xcm/src	72%	0%	➖
pallets/dapp-staking-migration/src	49%	0%	❌
pallets/ethereum-checked/src	75%	0%	➖
pallets/block-rewards-hybrid/src	91%	0%	✔
pallets/dapp-staking-v3/rpc/runtime-api/src	0%	0%	❌
pallets/dapp-staking-v3/src/test	0%	0%	❌
precompiles/unified-accounts/src	100%	0%	✔
precompiles/xvm/src	74%	0%	➖
pallets/unified-accounts/src	84%	0%	✔
chain-extensions/types/xvm/src	0%	0%	❌
pallets/astar-xcm-benchmarks/src/fungible	100%	0%	✔
chain-extensions/pallet-assets/src	56%	0%	➖
pallets/dapps-staking/src/pallet	86%	0%	✔
pallets/inflation/src	83%	0%	✔
chain-extensions/types/unified-accounts/src	0%	0%	❌
pallets/astar-xcm-benchmarks/src/generic	100%	0%	✔
precompiles/sr25519/src	64%	0%	➖
precompiles/assets-erc20/src	81%	0%	✔
pallets/xvm/src	51%	0%	➖
chain-extensions/xvm/src	0%	0%	❌
pallets/collator-selection/src	90%	0%	✔
primitives/src/xcm	66%	0%	➖
pallets/dynamic-evm-base-fee/src	92%	0%	✔
chain-extensions/types/assets/src	0%	0%	❌
primitives/src	59%	0%	➖
precompiles/substrate-ecdsa/src	74%	0%	➖
chain-extensions/unified-accounts/src	0%	0%	❌
pallets/dapp-staking-v3/src	87%	0%	✔
precompiles/dapps-staking/src	94%	0%	✔
pallets/dapp-staking-v3/src/benchmarking	98%	0%	✔
precompiles/dapp-staking-v3/src/test	0%	0%	❌
pallets/dapps-staking/src	90%	0%	✔
pallets/xc-asset-config/src	64%	0%	➖
pallets/static-price-provider/src	58%	0%	➖
precompiles/dapp-staking-v3/src	90%	0%	✔
Summary	79% (4378 / 5562)	0% (0 / 0)	➖

Minimum allowed line rate is 50%