[Bug]: logo missing from emails from the guest app

[salmart-dev]

Bug description

When emails are sent by apps that copy parts of the mail body instead of using useTemplate they miss the embedded logo.

Steps to reproduce

1. Enable the guests app v4.10
2. Share a file via mail selecting "Invite guest"
3. The email received by the guest is missing the logo

Expected behavior

The logo is visible

Nextcloud Server version

Reproducible on 34.0.4

Additional info

This is a regression caused by #62933, affects any app that does not use useTemplate to build mail bodies.