keep-tecdsa - withdraw should check for zero value transfer
Requesting the withdrawal of zero `ETH` in `KeepBonding.withdraw` should fail as this would allow the method to succeed, calling the user-provided destination even though the sender has no unbonded value.\\n```\\nfunction withdraw(uint256 amount, address payable destination) public {\\n    require(\\n        unbondedValue[msg.sender] >= amount,\\n        "Insufficient unbonded value"\\n    );\\n\\n    unbondedValue[msg.sender] -= amount;\\n\\n    (bool success, ) = destination.call.value(amount)("");\\n    require(success, "Transfer failed");\\n}\\n```\\n\\nAnd a similar instance in BondedECDSAKeep:\\n```\\n/// @notice Withdraws amount of ether hold in the keep for the member.\\n/// The value is sent to the beneficiary of the specific member.\\n/// @param \\_member Keep member address.\\nfunction withdraw(address \\_member) external {\\n    uint256 value = memberETHBalances[\\_member];\\n    memberETHBalances[\\_member] = 0;\\n\\n    /\\* solium-disable-next-line security/no-call-value \\*/\\n    (bool success, ) = tokenStaking.magpieOf(\\_member).call.value(value)("");\\n\\n    require(success, "Transfer failed");\\n}\\n```\\n
Require that the amount to be withdrawn is greater than zero.
```\\nfunction withdraw(uint256 amount, address payable destination) public {\\n    require(\\n        unbondedValue[msg.sender] >= amount,\\n        "Insufficient unbonded value"\\n    );\\n\\n    unbondedValue[msg.sender] -= amount;\\n\\n    (bool success, ) = destination.call.value(amount)("");\\n    require(success, "Transfer failed");\\n}\\n```\\n