Missing mailing tokens in text messages generated from html source

For html-only emails, CRM_Mailing_BAO_Mailing::getTemplates() creates a text template from the html source. In this case getTokens() doesn't find any text tokens because there is no text body to pull them from.

This bug causes mailing tokens (e.g.,

{mailing.viewUrl}

) to remain unsubstituted in text-only emails generated from html source.

Patch attached.