[10.x] Markdown Mailables: Allow omitting Footer and Header when customising components

[jorisnoo]

When the Markdown mail components are exported, it is possible to customise the templates and eg. to remove the header and footer components of the email templates. While they may be commented out in the mail/html/message.blade.php file, doing the same in the mail/text/message.blade.php file would throw an error since the mail/text/layout.blade.php file is expecting a $footer and $header variable to be set.

This admittedly very tiny change applies the current behaviour from the html template to the text version — by falling back to an empty string if the header/footer variables are not set.