Upstreaming from https://phabricator.wikimedia.org/T346876

Steps to reproduce:

1. Pretend you are a webcrawler which for no particular reason goes to https://we.phorge.it/?__path__=%2fsearch%2fhovercard%2f&cards={%22whatever%22:{%22whatever%22:%22foobar%22}}
2. As an admin, get an unneeded Unhandled Exception ("RuntimeException") Undefined index: objectPHID entry in your server error logs

(This is a bit similar to T15910 when it comes to passing random fuzzy data as URL parameters while the code doesn't check for their validity.)