fix: define array key for "allowComments"

[DmySyz]

Adds a missing key for the "allowComments". Otherwise when changing this in the configuration - a warning is triggered

"Undefined array key 'allowComments' at /var/www/nextcloud/apps/forms/lib/Controller/ConfigController.php#69"

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[Chartman123]

/backport to stable5.3

[github-actions]

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