fix(protocol-contracts): skip zero-amount stake/unstake operations (N-08)

[melanciani]

NOTE: as opposed to the audit report, I introduced a revert in unstake instead of an early return like in stake because it is unclear what releaseTime we should consider then. We'll first need to confirm this with auditors

• Add early return for zero-amount stake in ProtocolStaking
• Add revert with ZeroUnstakeAmount error for zero-amount unstake in ProtocolStaking
• Add revert with NoAssetsToWithdraw error in OperatorStaking.requestRedeem when no assets to withdraw
• Add tests for new behaviors

refs https://github.com/zama-ai/fhevm-internal/issues/826

[mergify]

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