Never generate $0.00 invoices

Summary: Ref T7769. It might possibly make sense to generate these some day, but for now they aren't payable and we have no use for them.

Test Plan:

• Generated $0.00 invoices, saw code decline to invoice.
• Generated a $7.99 invoice, got an invoice.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7769

Differential Revision: https://secure.phabricator.com/D12878