job parsing: fix panic when variable validation is missing condition

[tgross]

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.

[shoenig]

LGTM!