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