Time serie NULL visualization in multiple query

### What happened?

Context: time serie panel in grafana dashboard with multiple query coming from postgresql-datasource one with nulls and the other without them

timeserie visualization with both queries visible:

timeserie visualization with only query with nulls visible:

### What did you expect to happen?

I'd expect a consistent behavior. The correct visualization of the query with nulls is the second image shown and i expect the same visualization even adding other independent queries

### Did this work before?

No idea

### How do we reproduce it?

just import the panel exported below and try to deactivate the visualization of the query without nulls and you obtain the screenshots i posted

### Is the bug inside a dashboard panel?

[debug-TimeSeriesVisualization-2025-12-18 13_41_58.json.txt](https://github.com/user-attachments/files/24235572/debug-TimeSeriesVisualization-2025-12-18.13_41_58.json.txt)

### Environment (with versions)?

Grafana: 12.1
Browser: Google Chrome

### Grafana platform?

Docker

### Datasource(s)?

PostgreSQL

Start by importing the attached debug-TimeSeriesVisualization JSON in Grafana 12.1 with the PostgreSQL datasource and reproduce the time series panel with both queries enabled. Compare it with the view showing only the query containing NULLs; done means the NULL-containing series renders consistently when the independent query is added.

Written by the indexing model from the issue text.