casting job_config to QueryJobConfig if supplied by user

[parthmishra]

Small fix for #2612 by constructing a QueryJobConfig object from the job_config dict if it is given.

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[k8s-ci-robot]

Hi @parthmishra. Thanks for your PR.

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[parthmishra]

@googlebot I signed it!

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[parthmishra]

@googlebot I signed it!

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[numerology]

Thanks!
/ok-to-test

[hongye-sun]

/ok-to-test
/lgtm
/approve

Thanks for the fix.

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: hongye-sun

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[numerology]

/retest