[CONF] when in grouping rules some field has empty value is accepted as valid

[iariasleon]

Ex:

       {
            "id": 15,
            "fields": [
                "entityId"
            ],
            "regex": "destmissing(\\d*)",
            "destination": "",
            "fiware_service_path": "errordataset"
        },

[iariasleon]

LGTM

time=2015-06-19T14:39:31.896CEST | lvl=WARN | trans= | function=createGroupingRules | comp=Cygnus | msg=com.telefonica.iot.cygnus.interceptors.GroupingInterceptor[195] : Invalid grouping rule, some field is empty. It will be discarded. Details={"id":15,"fiware_service_path":"errordataset","regex":"destmissing(\\d*)","destination":"","fields":["entityId"]}