Require `_data.length` > 0

[hujw77]

https://github.com/darwinia-network/darwinia-messages-sol/blob/28ec79482a0d5ef500fe1ce066c51165f4efe75c/contracts/periphery/contracts/s2s/types/CommonTypes.sol#L81

[wuminzhe]

I'll fix it. Under what conditions will the data length be 0?

[wuminzhe]

the type of AssignedRelayers is Option<Vec<PalletFeeMarketRelayer>>.

• if AssignedRelayers is None: the storage data returned is null

• if AssignedRelayers is Some([...]) the storage data returned is scale bytes of type Vec<PalletFeeMarketRelayer>