CSV report (saved search@dashboard): improve the behavior of having zero fields selected in the saved search

[0asp0]

Kibana version: 7.3.1

Elasticsearch version: 7.3.1

Server OS version: Windows 10

Browser version: Vivaldi

Browser OS version: Windows 10

Original install method (e.g. download page, yum, from source, etc.): zip

Describe the bug:
If I do not set any field in my saved search, I expect all fields to be exported to csv, which are used in my result set. That way it is done by discovery csv export where it is whitelisting all used fields in the report object in kibana index.
But on dashboard level only the timestamp is exported to csv if no fields are selected.

Steps to reproduce:

1. save a search with no fields selected (_source will be displayed) as test2
2. add saved search test2 to a dashboard
3. export test2 by hitting Download CSV

Expected behavior:
All fields which are used in result set should be exported. That would be the same behavior like the csv export int discovery module.

But the csv export shows some json like output:
2019-08-23T12:57:05.371Z,"{""@timestamp"":""2019-08-23T12:57:05.371Z""}"

I expected to find all fields which are used in the csv file.

Screenshots (if relevant):

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context:
discussion:
https://discuss.elastic.co/t/feedback-on-csv-export-of-saved-search-in-dashboard/196748

[elasticmachine]

Pinging @elastic/kibana-stack-services

[tsullivan]

Duplicate of #18320