Linter will throw an error when a "Ad hoc filters" variable is used

[antoniocascais]

Bug

When we have a "Ad hoc filters" variable created, linter will exit with error Error: failed to parse dashboard: invalid type for field 'query': <nil>

Expected behaviour

The code should be able to handle "ad hoc filters" and not throw an error when they exist.

Steps to reproduce

Just create a variable "Ad hoc filters" in your dashboard and run the linter.
For example:

{
  "annotations": {
    "list": [
      {
        "builtIn": 1,
        "datasource": "-- Grafana --",
        "enable": true,
        "hide": true,
        "iconColor": "rgba(0, 211, 255, 1)",
        "name": "Annotations & Alerts",
        "target": {
          "limit": 100,
          "matchAny": false,
          "tags": [],
          "type": "dashboard"
        },
        "type": "dashboard"
      }
    ]
  },
  "editable": true,
  "fiscalYearStartMonth": 0,
  "graphTooltip": 0,
  "links": [],
  "liveNow": false,
  "panels": [
    {
      "fieldConfig": {
        "defaults": {
          "color": {
            "mode": "palette-classic"
          },
          "custom": {
            "axisLabel": "",
            "axisPlacement": "auto",
            "barAlignment": 0,
            "drawStyle": "line",
            "fillOpacity": 0,
            "gradientMode": "none",
            "hideFrom": {
              "legend": false,
              "tooltip": false,
              "viz": false
            },
            "lineInterpolation": "linear",
            "lineWidth": 1,
            "pointSize": 5,
            "scaleDistribution": {
              "type": "linear"
            },
            "showPoints": "auto",
            "spanNulls": false,
            "stacking": {
              "group": "A",
              "mode": "none"
            },
            "thresholdsStyle": {
              "mode": "off"
            }
          },
          "mappings": [],
          "thresholds": {
            "mode": "absolute",
            "steps": [
              {
                "color": "green",
                "value": null
              },
              {
                "color": "red",
                "value": 80
              }
            ]
          }
        },
        "overrides": []
      },
      "gridPos": {
        "h": 9,
        "w": 12,
        "x": 0,
        "y": 0
      },
      "id": 2,
      "options": {
        "legend": {
          "calcs": [],
          "displayMode": "list",
          "placement": "bottom"
        },
        "tooltip": {
          "mode": "single",
          "sort": "none"
        }
      },
      "targets": [
        {
          "datasource": {
            "type": "prometheus",
            "uid": "prometheus"
          },
          "exemplar": true,
          "expr": "",
          "interval": "",
          "legendFormat": "",
          "refId": "A"
        }
      ],
      "title": "Panel Title",
      "type": "timeseries"
    }
  ],
  "schemaVersion": 35,
  "style": "dark",
  "tags": [],
  "templating": {
    "list": [
      {
        "filters": [],
        "hide": 0,
        "name": "query0",
        "skipUrlSync": false,
        "type": "adhoc"
      }
    ]
  },
  "time": {
    "from": "now-6h",
    "to": "now"
  },
  "timepicker": {},
  "timezone": "",
  "title": "New dashboard",
  "version": 0,
  "weekStart": ""
}