bug: `OwnerAddress` field of response `token_security` api

[fenghaojiang]

This field owner address:
This contract's owner address. No value will be returned if the owner address is unknown. An empty sting will be returned if the contract has no owner.

But in the sdk it use type string to receive from a json value so it could be ambiguous.

// No return means unknown; Return empty means there is no ownership or can't find ownership.(Notice:(1) When "is_open_source": "0", there will be no return.

This PR aims to fix the case that former version can not identify these two situations:

no return
return empty
In former version these two situation would exactly be the same. So type of OwnerAddress should be pointer.

OwnerAddress string `json:"owner_address,omitempty"` 

Should be:

OwnerAddress *string `json:"owner_address,omitempty"`