Summary: Check that we have objects with PHIDs before running a query. If there are none, return an empty array instead. ``` EXCEPTION: (Exception) Edge list passed to "withSourcePHIDs(...)" is empty, but it must be nonempty. at [<phorge>/src/infrastructure/edges/query/PhabricatorEdgeQuery.php:50] ``` Closes T15789 Test Plan: * Log into Phorge, go to http://phorge.localhost/maniphest/query/assigned/ and make sure you have zero assigned tasks. * Click "Use Results" and select "Export Data". * In the "Export Results" dialog, keep default Format = csv and click "Download Data". * After applying the patch, get the expected dialog "Download File - Download file assigned.csv (264 B)?" and resulting file with only column headers and no data, instead of an unhandled exception. Reviewers: O1 Blessed Committers, speck Reviewed By: O1 Blessed Committers, speck Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno Maniphest Tasks: T15789 Differential Revision: https://we.phorge.it/D25613