Remove heading in swiftmailer email template when heading it is empty

Problem/Motivation

There is an extra comma rendered in email template for which user greetings are disabled.

Steps to reproduce

- Install Open Social
- Go to admin/config/opensocial/swiftmail and add for example "register_no_approval_required" under "Disabled email message keys with user greeting"
- Sign up with new account so you will trigger sending of email with template "register_no_approval_required"
- The email will be send with an extra comma just below the header

Proposed resolution

Do not render heading in email template when it is empty.

Remaining tasks

This changes need to be applied also for related issue #3259374: Create a new template for the new email engine

User interface changes

N/A

API changes

N/A

Data model changes

N/A