Allowing a name to be an empty string (develop branch)

[Buzhanin]

This is a clone of !790 created for a develop branch

Change Description

In order to update owner permission of an account the name of it's parent permission should be set to an empty string

Example:

{
    "account": "eosio",
    "name": "updateauth",
    "authorization": [{
        "actor": "someaccount"
        ",
        "permission": "owner"
    }],
    "data": {
        "account": "someaccount",
        "permission": "owner",
        "parent": "",
        "auth": {
            "threshold": 1,
            "keys": [{
                "key": "NEW_PUBLIC_KEY",
                "weight": 1
            }],
            "accounts": [],
            "waits": []
        }
    }
}

Current regex validation doesn't allow fields of name type to be an empty string. It causes a fail in serialisation of above mentioned action.