If you try to unset the ownerPHID on a maniphest task via conduit, the api will throw ERR-NO-EFFECT as it checks if there are any non-null values to save.
echo '{"id": 48, "ownerPHID": null}' | arc call-conduit maniphest.update Waiting for JSON parameters on stdin... {"error":"ERR-NO-EFFECT","errorMessage":"ERR-NO-EFFECT: Update has no effect.","response":null}
D10586 would profit from a fix.
Maybe either allow 'null' explicitly for some fields or provide a special PHID that stands for NULL ?