fix: Allow a null label in LabelEventSchema

[pataar]

Problem

The resource label events endpoints (IssueLabelEvents, MergeRequestLabelEvents, EpicLabelEvents) return "label": null for an event whose label has since been deleted:

{
  "id": 123,
  "user": { "id": 1, "username": "jane", "...": "..." },
  "created_at": "2024-01-29T14:26:49.909+01:00",
  "resource_type": "Issue",
  "resource_id": 456,
  "label": null,
  "action": "add"
}

GitLab keeps the event and nullifies its label reference when the label is removed from the project or group, so any long-lived issue or merge request can carry a few of these. Observed on a self-managed GitLab instance.

LabelEventSchema types the field as label: LabelSchema, so event.label.name compiles without complaint and throws TypeError: Cannot read properties of null (reading 'name') at runtime.

Change

label: LabelSchema → label: LabelSchema | null.

Types only, no runtime change. Nothing inside gitbeaker reads the field. For consumers this surfaces the missing null check at compile time — which is the point, but it does mean code doing event.label.name will need a ?. after upgrading.