Only validate form subtype edits if subtype transactions are present

Summary: Fixes T12347. Ref T12314. Validation gets called no matter what, but is only relevant if the form supports subtypes.

Test Plan: Marked/unmarked a Paste form as editable.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12347, T12314

Differential Revision: https://secure.phabricator.com/D17457