TSVB wrong default date field on default index-pattern

[markov00]

Kibana version:
master

Describe the bug:
When creating a fresh TSVB visualization, the default index pattern is used.
The first data query is then issued with that default index-pattern against what should be the default date field. I said "should be" because if I'm not currently seeing this behaviour if I'm using the kibana_sample_data_ecommerce that has the order_date as default date field instead of the standard @timestamp.

Steps to reproduce:

1. set kibana_sample_data_ecommerce as default index pattern
2. create a new TSVB visualization
3. you will see a flat green line on the timeseries visualization (check api/metrics/data API call via devtools, the panel object is issuing a that call with the @timestamp wrong date field.
4. now click on the Panel Option and the visualization will disasppear showing an error message The request for this panel failed
5. check the last api/metrics/data call, the time_field value is null and ES is complaining about [date_histogram] field doesn't support values of type: VALUE_NULL

Expected behavior:

The default index pattern should use the default time field specified on that patter (that is correctly picked up by the api/metrics/field call but not used on the /data call.

Screenshots (if relevant):

Any additional context:

First request with wrong time_field:

{
  "timerange": {
    "timezone": "Europe/Rome",
    "min": "2018-05-08T08:49:58.829Z",
    "max": "2019-05-08T08:49:58.830Z"
  },
  "query": [{ "query": "", "language": "kuery" }],
  "filters": [],
  "panels": [
    {
      "id": "61ca57f0-469d-11e7-af02-69e470af7417",
      "type": "timeseries",
      "series": [
        {
          "id": "61ca57f1-469d-11e7-af02-69e470af7417",
          "color": "#68BC00",
          "split_mode": "everything",
          "metrics": [
            { "id": "61ca57f2-469d-11e7-af02-69e470af7417", "type": "count" }
          ],
          "separate_axis": 0,
          "axis_position": "right",
          "formatter": "number",
          "chart_type": "line",
          "line_width": 1,
          "point_size": 1,
          "fill": 0.5,
          "stacked": "none"
        }
      ],
      "time_field": "@timestamp",
      "index_pattern": "",
      "interval": "auto",
      "axis_position": "left",
      "axis_formatter": "number",
      "axis_scale": "normal",
      "show_legend": 1,
      "show_grid": 1,
      "default_index_pattern": "kibana_sample_data_ecommerce"
    }
  ],
  "state": {}
}

subsequent wrong call with null field

{
  "timerange": {
    "timezone": "Europe/Rome",
    "min": "2018-05-08T08:50:09.412Z",
    "max": "2019-05-08T08:50:09.412Z"
  },
  "query": [{ "query": "", "language": "kuery" }],
  "filters": [],
  "panels": [
    {
      "id": "61ca57f0-469d-11e7-af02-69e470af7417",
      "type": "timeseries",
      "series": [
        {
          "id": "61ca57f1-469d-11e7-af02-69e470af7417",
          "color": "#68BC00",
          "split_mode": "everything",
          "metrics": [
            { "id": "61ca57f2-469d-11e7-af02-69e470af7417", "type": "count" }
          ],
          "separate_axis": 0,
          "axis_position": "right",
          "formatter": "number",
          "chart_type": "line",
          "line_width": 1,
          "point_size": 1,
          "fill": 0.5,
          "stacked": "none"
        }
      ],
      "time_field": null,
      "index_pattern": "",
      "interval": "auto",
      "axis_position": "left",
      "axis_formatter": "number",
      "axis_scale": "normal",
      "show_legend": 1,
      "show_grid": 1,
      "default_index_pattern": "kibana_sample_data_ecommerce"
    }
  ],
  "state": {}
}

cc @elastic/kibana-tsvb-external