R1:84c57f5e5751

Fix exception trying to export empty task list to CSV

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.
*…