Better failure if there are no variables

Here's an initial attempt. I've been running this job but cannot get the rules: to trigger correctly yet.

Great idea on the ".pre" job. At first I thought it was just invalid syntax, but yeah, having just variables: seems to be valid syntax under GitLab CI standards, but the side effects of it are totally not expected (empty variables!!).

The trick to getting the rules: criteria working properly was to check against null not an empty string. It sounds obvious now, but I don't think we have ever tested a variable for null before in the templates. The conditions =="" and !="" were both evaluating to false, so I did some research on what it should be checking for, and found null

Test with variables: followed by no variable definitions:
https://git.drupalcode.org/project/scheduler/-/pipelines/305371

After removing the empty variables: line
https://git.drupalcode.org/project/scheduler/-/pipelines/305375

Here's the job using 'alpine' image. Worked fine.
https://git.drupalcode.org/project/scheduler/-/jobs/3004765

Looks pretty good already. Just added some minor feedback to expand the if condition a little to match the printf values that we show.

I've responded in the MR and this is ready for review.
Here's a re-run of the test https://git.drupalcode.org/project/scheduler/-/pipelines/306428

Looks good. Thanks for addressing the feedback. RTBC.

Merged! Thanks.

Automatically closed - issue fixed for 2 weeks with no activity.