Check for nil values when parsing HCL strings

[Juanadelacuesta]

Nomad panics when parsing a job specification which contains a kill_timeout that is null.
This is normally caught with a validation error, but if the null is provided via a ternary expression false ? "5s" : null Nomad panics instead.

It fixes 25292

I couldn't find any other places where not checking the nil value could lead to a panic.

Description

Testing & Reproduction steps

Links

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[tgross]

LGTM!

Tested with both the job plan command and the Parse API:

$ nomad job plan /tmp/example.nomad.hcl
Error getting job struct: Error parsing job file from /tmp/example.nomad.hcl:
example.nomad.hcl:11,22-27: Unsuitable value; Unsuitable duration value: nil

$ curl \
    --request POST \
    --data @payload.json \
    http://localhost:4646/v1/jobs/parse

Failed to parse job: input.hcl:11,22-27: Unsuitable value; Unsuitable duration value: nil%