R1:e556d205772a

Fix some issues where Conpherence would make to many draft requests

Summary:
A few minor fixes:

- When we build a tag with `"meta" => null`, strip the attribute like we do for all other attributes. Previously, we would actually set the metadata to `null`. This happened with the Conpherence form.
- Just respond to the draft request with an empty (but valid) response, instead of building a dialog.
- `PhabricatorShapedRequest` is confusingly named and I should have caught this in review, but the basic…