feat: Print email metadata on preview without body

[pfac]

Why:

• In a setup where one uses a email service with transactional templates or any other mechanism where the system does not actually render the email content but instead triggers an external service to generate the emails, previewing sent emails in a development environment is pretty much useless since nothing is shown. Instead developers have to use the logs to try and parse the relevant email information.

This change addresses the issue by:

• Including the email information as metadata when the email has no body. This shows the data being sent to external service to generate the email, thus providing a way for developers to check the relevant values in a development environment.

BEGIN_COMMIT_OVERRIDE feat: Print email metadata on preview without body END_COMMIT_OVERRIDE

[pfac]

Just noticed this was closed by accident when we merged the change locally.

[pfac]

Ping, is anyone looking at these?

[doomspork]

@pfac yes but the team is very small so please bear with us.

As for the change: I understand why this is helpful, I've run into something similar before, but I'm also not sure if this is the right solution 🤔 Something about throwing an inspect in the UI gives me pause. I'd love to see if others from @beam-community/team have input.