the array of contacts all that is return or are there any other fields on getContacts.data? (The state tab of debug tools can be particularly useful here as well)
If it's just the array you might try something like {{ getContacts.data.length === 50}} in the "Has next page" field. This assumes that the last page will return <50 records. It's a bit imperfect though because if you have some multiple of 50 records you need to be able to fetch a blank page as the last page.
I'm a little worried since it seems like requesting a page that doesn't exist just returns the last page - can you confirm whether or not that's the case?
Again, ideally, the API would have another property besides the records that indicates whether or not there's another page.