fix: upgrade nodemailer and add validation for mail transports

[NicolasMarino]

I’ve been using @adonisjs in my app (conectandote.com.uy) for over a year now. Recently, I had to implement a few fixes locally to keep things stable and I’d love to contribute them back to the repo.

Basically, I worked on:

Nodemailer upgrade: Bumped it to the latest version to get the latest security patches.

Stability fixes: Added some missing validation to the Message class and Mailgun transport. I noticed the app would sometimes crash with a TypeError if certain config values or recipients ended up being undefined.

I've already ran the tests locally and everything is looking green, no regressions found.

I have the branch ready to go. Should I open a PR from my fork so you guys can take a look?