`gppa-use-choice-labels-when-populating-from-another-field.php`: Fixed PHP 8 warnings.

[saifsultanc]

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Context

⛑️ Ticket(s): https://secure.helpscout.net/conversation/2205739488/46442?folderId=3808239

Summary

PHP8 warnings were noticed in the investigation of the ticket.

A before and after overview of the warnings (and confirmation that the functionality remains intact):

https://www.loom.com/share/667f4e2a5de045fe9290cca33a17710a