[TSVB] More than one empty labels in markdown breaks the values for all other labels.

[marius-dr]

Kibana version: 7.13.0 Snaphsot

Describe the bug:
If you have a field with empty labels, it used to break markdown completely. #90756 This has been fixed in #91838 by now showing (empty) instead of blank.
If you add only one empty label, everything works as intended, if you add more than one all variables are converted to their code representation,

Steps to reproduce:

1. Create a markdown on a series with split by terms (where you have empty values for the field in the split). For example: kibana_sample_data_logs and split by extension.keyword.
2. Add 2 or more (empty) variables on the markdown.

Screenshots (if relevant):
With 1 "empty" variable:

With 2 "empty" variables:

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)