frontend: aggregate functions on missing label-values return a `null` result instead of `[]`

Hi folks.

I noticed a small difference between the behaviour of OSS Prometheus and the frontend component in the `query_range` API:

* Given a metric `foo` with label values `bar="0"` and `bar="1"`
* If I run an aggregate query on a non-existent label-value like `count(foo{bar="2"})`,
* The result in OSS Prometheus is:
* `{"status":"success","data":{"resultType":"matrix","result":[]}}`
* The result in prometheus-engine's frontend is:
* `{"status":"success","data":{"resultType":"matrix","result":null}}`

This causes the frontend to break (query ui disappears; screen becomes empty) and causes issues for some prometheus tools like pint.

Is it possible to preserve compatibility and return an empty array in this case instead of `null`?