[Alerting] Enable updating system actions through the Bulk Edit Rule …

…API (#168226)

Summarize your PR. If it involves visual changes include a screenshot or
gif.

Depends on: #167871,
#167884

### Checklist

Delete any items that are not applicable to this PR.

- [x] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios

### For maintainers

- [x] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

---------

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
Co-authored-by: Julia Guskova <iuliia.guskova@elastic.co>

x-pack/plugins/alerting/common/routes/rule/apis/bulk_edit/schemas/v1.ts

@@ -38,7 +38,7 @@ const ruleSnoozeScheduleSchemaWithValidation = schema.object(
 );
 
 const ruleActionSchema = schema.object({
-  group: schema.string(),
+  group: schema.maybe(schema.string()),
   id: schema.string(),
   params: schema.recordOf(schema.string(), schema.any(), { defaultValue: {} }),
   uuid: schema.maybe(schema.string()),