[DUOS-2884][risk=no] Bug Fix: Ensure users exist for sending submitted dataset emails

[rushtong]

Addresses

https://broadworkbench.atlassian.net/browse/DUOS-2884

Summary

• Minor bug fix to get chairs for each DAC when sending submitted dataset emails.

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[aarohinadkarni]

👍

[fboulnois]

Thanks for catching this. I wonder if we can add a test for this issue?