[TSVB] Group by Terms functionality not working propertly

[alexwizp]

Regression of #36784

Kibana version:
master branch

Steps to reproduce:

1. Create new TSVB visualization or open existing one (index pattern should be empty)
2. Set 'Terms' value for Group By combobox
3. Try to expand 'By' combobox

Expected behavior:
'By' combobox should allow us select fields related to the default index pattern.

Current behavior:
Combobox is empty

Root cause:

[elasticmachine]

Pinging @elastic/kibana-app

[TinaHeiligers]

@alexwizp, if using the default_index_pattern in the render method is a problem, what index pattern should we be using, if any at all? i.e., should we be using:
const indexPattern = (model.override_index_pattern && model.series_index_pattern) || panel.index_pattern only?

[alexwizp]

I think we should use value of the 'default index pattern' only for one case, it's a showing this value in a placeholder for 'Index Pattern' input. In other cases we should work with empty string value.

[TinaHeiligers]

@alexwizp: The bug is fixed in PR 39723.