[TSVB] [Markdown] markdown section do not render after change data parameter

[vitalics]

Kibana version:
8.0.0, 7.x
Elasticsearch version:
8.0.0
Server OS version:

Browser version:

Browser OS version:

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

Describe the bug:
markdown section does not render after the set variable name. As a result, you can see that issue(see screen below)

the DOM does not appear, it seems some of the condition return null

Steps to reproduce:

1. create TSVB visualization
2. moving on the markdown section
3. on the markdown section moving on data section and set the variable name for the first series
   4 switch from data to markdown section

Expected behavior:
Markdown editor and table should be rendered and appear on DOM
Screenshots (if relevant):

Errors in browser console (if relevant):

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

Any additional context:
This bug was caught by failed test #41452

[elasticmachine]

Pinging @elastic/kibana-app

[alexwizp]

regression of #37185