uncomment MAILER_DSN env for symfony/mailer

[kbond]

Q	A
License	MIT
Doc issue/PR	n/a

Currently, when installing symfony/mailer, the MAILER_DSN is commented out so you get an error when trying to send an email. I feel this is a bit of a DX bummer.

I think it should default to null://null so you can start using it right away (preview in the profiler and use in your tests).

Any bridge you install for production sending would configure MAILER_DSN elsewhere (ie secrets or .env.local).

[github-actions]

Thanks for the PR 😍

How to test these changes in your application

1. Define the SYMFONY_ENDPOINT environment variable:

   # On Unix-like (BSD, Linux and macOS)
   export SYMFONY_ENDPOINT=https://raw.githubusercontent.com/symfony/recipes/flex/pull-1352/index.json
   # On Windows
   SET SYMFONY_ENDPOINT=https://raw.githubusercontent.com/symfony/recipes/flex/pull-1352/index.json

2. Install the package(s) related to this recipe:

   composer req 'symfony/flex:^1.16'
   composer req 'symfony/mailer:^4.3'

3. Don't forget to unset the SYMFONY_ENDPOINT environment variable when done:

   # On Unix-like (BSD, Linux and macOS)
   unset SYMFONY_ENDPOINT
   # On Windows
   SET SYMFONY_ENDPOINT=

Diff between recipe versions

In order to help with the review stage, I'm in charge of computing the diff between the various versions of patched recipes.
I'm going keep this comment up to date with any updates of the attached patch.

[fabpot]

It makes sense to me.