Fix incorrect nullness in FlinkJobInvoker and JobInvoker

[kennknowles]

Note that this is an actual error not just a hardening of the codebase: the retrievalToken is marked as required in the proto.

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[kennknowles]

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Reminder, please take a look at this pr: @robertwb

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Reminder, please take a look at this pr: @robertwb

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Assigning new set of reviewers because Pr has gone too long without review. If you would like to opt out of this review, comment assign to next reviewer:

R: @robertwb added as fallback since no labels match configuration

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[derrickaw]

Hi @robertwb, please review when you have a chance. Thanks!

[github-actions]

Reminder, please take a look at this pr: @robertwb

[github-actions]

Assigning new set of reviewers because Pr has gone too long without review. If you would like to opt out of this review, comment assign to next reviewer:

R: @robertwb added as fallback since no labels match configuration

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[kennknowles]

R: @je-ik

[kennknowles]

I don't have time that I thought I would have to work on FlinkRunner right now, but I figure might as well get this tiny improvement checked in.

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[je-ik]

LGTM, thanks!

[Abacn]

This has been reverted in #34628 attempt to fix #31418 . The reason of breakage isn't clear but this appears to be the only flink runner related changes in the time range