[TSVB] Color rules are not applied for null series

[stratoula]

Kibana version:
7.12+

Describe the bug:
By stop inserting zeroes for null series (#90861), we introduced a bug. Specifically, all the color rules that try to add a color rule for less than 0 are not applied.

Steps to reproduce:

1. Create a TSVB metric viz
2. Use the kibana_sample_data_logs dataset without any change
3. I used a timerange of the last 12 minutes in order to have null series
4. Apply a color rule (change the background color for metric less than or equal to 0
5. The color is not applied

Expected behavior:
Users expect the color rule to work

Screenshots (if relevant):

[elasticmachine]

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

[alexwizp]

I have one suggestion to implement it in a little bit different way. This solution should be more flexible
Let's:

1. add new rule e.g. empty
   
2. create a migration script to fix all broken TSVB visualizations by adding a new empty rule. I think it should work for both less/greater than or equal : 0