`nil` dereference in webhooks

[KN4CK3R]

          Unfortunately this PR broke Gitea for us as the call to `p.Repository` inside `func (m *MatrixPayload) Package(p *api.PackagePayload) (api.Payloader, error)` (`services/webhook/matrix.go:239`) results in a nil pointer dereference. Looks like `p.Repository` is not set when triggered from `UploadManifest`. Stack trace (Gitea 1.21):

/source/services/webhook/matrix.go:215 (0x23cf774)
/source/services/webhook/payloader.go:58 (0x23dae5d)
/source/services/webhook/matrix.go:239 (0x23cfac4)
/source/services/webhook/webhook.go:198 (0x23dfc6b)
/source/services/webhook/webhook.go:261 (0x23e0435)
/source/services/webhook/notifier.go:882 (0x23da6d0)
/source/services/webhook/notifier.go:863 (0x23da495)
/source/services/notify/notify.go:353 (0x2306326)
/source/routers/api/packages/container/manifest.go:309 (0x23062da)
/source/routers/api/packages/container/manifest.go:184 (0x2305364)
/source/routers/api/packages/container/manifest.go:191 (0x230483c)
/source/routers/api/packages/container/manifest.go:75 (0x2304724)
/source/routers/api/packages/container/container.go:546 (0x2301bec)

Originally posted by @niklaskorz in #27839 (comment)