DEV-48075 - Fix missing warning from NoData in prometheus converter

[yasmin-tr]

This fix is based on the following:
grafana#93936

It was fixed on a later version on new prometheus client, but relevant for the current one as well.

The Issue
When running queries that return no data , but has warnings attached to it, we will not see the warning.
Even though the resulsts from datasource do contain a warning:

We expect to see:

But instead not seeing any warning, only No Data.
This is confusing since you don't know if there's an actual problem with missing data or the query is too big.

Root Cause
In the prometheus converter we are adding warnings to response on the response frames.
But in case of no-data there are no frames in response, causing nothing to be added.

Suggested Fix
Fix is pretty simple - just add the case where there are no frames in response and create a new frame for warnings and add it there.

[yasmin-tr]

run-test

[yasmin-tr]

run-test