lint.go: Handle variable template values

[bosc0]

Since the merge of #45 the linter breaks on jsons like this:

          {
            "allValue": ".+",
            "current": { },
            "datasource": "$datasource",
            "hide": 0,
            "includeAll": true,
            "label": "job",
            "multi": true,
            "name": "job",
            "options": [ ],
            "query": "label_values(awx_system_info, job)",
            "refresh": 1,
            "regex": "",
            "sort": 1,
            "tagValuesQuery": "",
            "tags": [ ],
            "tagsQuery": "",
            "type": "query",
            "useTags": false
         },

It will return an error: Error: failed to parse dashboard: 'text' property required even though it is valid in grafana. The handling of "current": { }, is the part that causes the error, this PR fixes this behaviour.

[rgeyer]

This is excellent, thank you! I actually encountered this as an issue while working on several mixins, so this saved me a lot of time :)