Alerting: Fix InfluxDB no data returns a frame

[grobinson-grafana]

What this PR does / why we need it:

This is one of two possible fixes for #55085. It changes the InfluxDB plugin to match the behaviour of the Prometheus plugin such that queries that return no data will return no frames instead of a frame with no fields.

Which issue(s) this PR fixes:

Fixes #55085

Special notes for your reviewer:

[grafanabot]

Drone build failed: https://drone.grafana.net/grafana/grafana-enterprise/33898

[kylebrandt]

I think we need to go the other way here and keep the single frame -- targeting that by v10 (data plane contract -- https://github.com/grafana/grafana-plugin-sdk-go/blob/main/data/contract_docs/contract.md#no-data-and-empty).

The primary reason for this is Frame Metadata, in particular we need it for information returned by the backend about queries that were made:

• ExecutedQueryString (post "raw/text query" macro expansion in various datasources)
• More future - the type identifier, so we in some cases we may know that

(Then reduce should handle NoData inputs that have a frame attached instead of erroring on them). Been a bit since I thought about that so not sure of exact behaviors, But NoData into a reduce probably also returns NoData (and then math probably needs to handle it to).

[grobinson-grafana]

Thank you! I wasn't sure if there was a purpose behind returning a frame with no fields, but I understand its for the metadata!