Backport of job parsing: fix panic when variable validation is missing condition into release/1.2.x

[hc-github-team-nomad-core]

Backport

This PR is auto-generated from #16018 to be assessed for backporting due to the inclusion of the label backport/1.2.x.

The below text is copied from the body of the original PR.

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Fixes #12513

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Test using the example jobspec from #12513:

$ nomad job run -var 'image_id=foo-' example.nomad
Error getting job struct: Error parsing job file from example.nomad:
example.nomad:5,14-14: Missing required argument; The argument "condition" is required, but no definition was found.
example.nomad:5,3-13: Invalid variable validation specification; validation requires a condition.