Thresholds: Saving dashboard emits or purges unnecessary `"value": null` in JSON

### What happened? It's been so many times that I see these:   ### What did you expect to happen? The appearance of `"value": null` should stay consistent. Either it's present all the time, or it's absent when it could be `null`. I think this is a fix: ```diff diff --git a/pkg/kinds/dashboard/dashboard_spec_gen.go b/pkg/kinds/dashboard/dashboard_spec_gen.go index 60ac16f3e..37f55c2ad 100644 --- a/pkg/kinds/dashboard/dashboard_spec_gen.go +++ b/pkg/kinds/dashboard/dashboard_spec_gen.go @@ -857,7 +857,7 @@ type Threshold struct { // Value represents a specified metric for the threshold, which triggers a visual change in the dashboard when this value is met or exceeded. // Nulls currently appear here when serializing -Infinity to JSON. - Value *float32 `json:"value"` + Value *float32 `json:"value,omitempty"` } // Thresholds configuration for the panel ``` ### Did this work before? AFAICT, never since 9.3.x (when I started using Grafana). ### How do we reproduce it? Find a panel and change some settings randomly. Unfortunately, I have yet to find a consistent way to trigger the addition or removal of these `"value": null` lines. ### Is the bug inside a dashboard panel? _No response_ ### Environment (with versions)? - Grafana: v10.4.3 (0bfd547800) - OS: Windows 10 Version 22H2 (OS Build 19045.4651) (I believe this is irrelevant) - Browser: Microsoft Edge 126.0.2592.113 (Official build) (64-bit ### Grafana platform? Docker ### Datasource(s)? _No response_