fix: Webhook request parsing

[anloubie2]

When a webhook was modified or an alert was sent, a runtime error: invalid memory address or nil pointer dereference was raised at the moment the alert was parsed to create the webhook message:

github.com/uptrace/uptrace/pkg/alerting.NewWebhookMessage(0xc0004bcc60, {0x3613b20?, 0xc0005cd0e0?}, {0x17b0660, 0xc001d66900})
        github.com/uptrace/uptrace/pkg/alerting/notif_channel_handler.go:565 +0x57
github.com/uptrace/uptrace/pkg/alerting.notifyByWebhookChannel({0x3610a20, 0xc001d66660}, 0xc0004bcc60, 0x40?, {0x3613b20?, 0xc0005cd0e0?}, 0xc001d665d0)
        github.com/uptrace/uptrace/pkg/alerting/notif_webhook.go:58 +0x185
github.com/uptrace/uptrace/pkg/alerting.(*NotifChannelHandler).sendWebhookTestMsg(0xc0012d52b0, {0x3610a20, 0xc001d66660}, 0xc001c7d200, 0xc001d665d0)
        github.com/uptrace/uptrace/pkg/alerting/notif_channel_handler.go:476 +0x206
github.com/uptrace/uptrace/pkg/alerting.(*NotifChannelHandler).WebhookUpdate(0xc0012d52b0, {0x360d250, 0xc00343bb80}, {0xc001455b00, {{0xc000044c34, 0x37}, 0xc000749980, 0xc0012d5a20, 0x0}})
        github.com/uptrace/uptrace/pkg/alerting/notif_channel_handler.go:442 +0x232

By changing the way the Event inside the alert is accessed, the problem doesn't show anymore

[vmihailenco]

Thank you 👍