Don't try to access the Stripe object until the user submits the credit card form

Summary: Ref T12232. I can't reproduce the original issue, but this should probably fix it without side effects?

Test Plan: Added a card with Stripe, but I could do that before too.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12232

Differential Revision: https://secure.phabricator.com/D17333