/tx/:txid/outspend/:vout returns 200 when :txid doesn't exist

```
$ curl -v https://blockstream.info/api/tx/0000000000000000000000000000000000000000000000000000000000000000/outspend/0
< HTTP/2 200
< server: nginx
< date: Tue, 01 Jun 2021 00:47:42 GMT
< content-type: application/json
< content-length: 15
< cache-control: public, max-age=10
< access-control-allow-origin: *
< access-control-expose-headers: x-total-results
< via: 1.1 google
< alt-svc: clear
<
* Connection #0 to host blockstream.info left intact
{"spent":false}
```