Add default value for config source on input defaults for recipes

Re-titling for clarity.

We could implement this by replacing the unconditional \RuntimeException with this:

if ($config->isNew()) {
  return $settings['value'] ?? throw new \RuntimeException("The '$name' config object does not exist.");
}

Nice one-line fix and easy to test. I'd be okay with making this allowance for the config value source in particular.

I created a MR with the change. Added Needs tests tag is someone wants to update/add a test for this.

Ready for a review, the nightwatch failure doesn't seem to be related.

This looks good to me but I don't think we need to bring the console-specific input collection stuff into it.

Updated issue summary.

We're going to park this for @alexpott to review.

I think we also should support the fallback value for env too. The differences between the env and the config implementations are interesting. Env with return an empty string when the env var does not exist but config will throw an exception. I wonder what the right the thing to do here is...

FWIW the concept of a fallback value seems go to me.

I like the idea of making it consistent.

Looks great, Love the additional test and the comment updates.

I added a draft change record and am moving to RTBC.

I think we should allow NULL as a fallback value. See review on MR.