Improve job fixutre

What does this MR do and why?

This MR slightly improves the jobs fixture by adding an additional check to ensure that data is returned and nodes are not nil. During development, the existing check expect_graphql_errors_to_be_empty was expected to warn if no data was returned, indicating an issue with the query or some other problem. With this improvement, the aim is to ensure that if no data is returned where it should be, the test fails.

discussion (context)

🛠 with ❤ at Siemens

How to set up and validate locally

1. Run bin/rails spec/frontend/fixtures/jobs.rb

MR acceptance checklist

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