Pay missingAccountFunds during estimation

[0xSulpiride]

We've just come accross an issue that Kernel accounts revert with the AA21 didn't pay prefund error during the estimation phase - when calling simulateHandleOps method of EntryPoint - although the sender had enough funds.

I believe this is because Kernel.validateUserOp requires a signature to be valid before sending missingAccountFunds.

kernel/src/Kernel.sol

Lines 145 to 154 in 50bd922

	} else {
	return SIG_VALIDATION_FAILED;
	}
	if (missingAccountFunds != 0) {
	assembly {
	pop(call(gas(), caller(), missingAccountFunds, 0, 0, 0, 0))
	}
	//ignore failure (its EntryPoint's job to verify, not account.)
	}
	userOp.signature = userOpSignature;

I'd suggest that Kernel.validateUserOp pays missingAccountFunds regardless of a signature otherwise it hurts UX (because we would be asking users for 2 signatures to send 1 userop)

Reference:
https://github.com/eth-infinitism/account-abstraction/blob/0a9d55ca73cd309f4ec1cda28284d2d039e552b9/contracts/core/BaseAccount.sol#L42-L48