Fix creating work item at group level with group_webhooks

What does this MR do and why?

Creating a work item at the group level would fail if the group has the group_webhooks feature available and at least one group scope webhook enabled. Hook data builder assumed work items always belong to a project and now that's not the case.

How to set up and validate locally

Steps required to validate this fix locally are described in Creating a group level work item fails if `grou... (#420167 - closed)

The group where you are creating the work item needs to have the group_webhooks feature available and at least one webhook for the group with GroupHook.new(group: group, issues_events: true)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #420167 (closed)