A request to /validator-exits-to-prepare without operator specified does return 10 validators:
http://KAPIURL:3600/v1/modules/1/validators/validator-exits-to-prepare/

For example:
https://holesky.beaconcha.in/validator/1461263

I would expect that a request to that endpoint returns zero validators or an error if an invalid or no operator id is given.

A request to an operator that does not exist returns zero validators:
http://KAPIURL:3600/v1/modules/1/validators/validator-exits-to-prepare/88899