R1:a8f937d3138a

Only add the Mail "STAMPS" body section if there are stamps

Summary: Ref T13053. Some mail (like push notification mail) doesn't currently generate any stamps. Drop this section if there aren't any stamps on the mail.

Test Plan: Will check push mail in production.

Maniphest Tasks: T13053

Differential Revision: https://secure.phabricator.com/D19028