Prevent slurping for some cases. (tldraw#5272)

Make sure we don't slurp into files that we don't own and that we don't
overwrite existing data.

### Change type
- [x] `improvement`

apps/dotcom/client/src/tla/components/TlaEditor/PreviewWelcomeDialog.tsx

@@ -76,7 +76,10 @@ function WelcomeDialog() {
 	])
 
 	if (data.loading) return null
-	const offerSlurp = data.ok && data.value
+	const file = app.getFile(fileId)
+	const isOwner = file && file.ownerId === app.getUser().id
+	const isEmpty = editor.store.allRecords().filter((r) => r.typeName === 'shape').length === 0
+	const offerSlurp = data.ok && data.value && isEmpty && isOwner
 
 	return (
 		<div>