Download csv from dashboard panel returns empty

[tzachshabtay]

Kibana version: 7.3.0

Elasticsearch version: 7.3.0

Server OS version: centos:7

Browser version: Chrome Version 76.0.3809.87 (Official Build) (64-bit)

Browser OS version: MacOS Mojave (Version 10.14.4)

Original install method (e.g. download page, yum, from source, etc.): docker.elastic.co/kibana/kibana:7.3.0

Describe the bug: Following the instructions in #34571, I enabled xpack.reporting.csv.enablePanelActionDownload: true in the config.
The Download csv... button now appears. For saved searches on index patterns with no time column defined the csv is generated fine. However for saved searches on index patterns with a time column the csv files are generated empty (the headers are there but no records, even though I do see rows in the dashboard).

Steps to reproduce:

1. Set xpack.reporting.csv.enablePanelActionDownload: true in config
2. Create an index pattern with a time column
3. Create a saved search on that index pattern
4. Create a dashboard with that saved search
5. Filter the time so that the dashboard have rows
6. Click on "..." for the saved search panel, and then on "Download csv"

Expected behavior:

The csv should have the same rows that the dashboard has.

@joelgriffith @tsullivan

[elasticmachine]

Pinging @elastic/kibana-stack-services