Resolve "Submit error with empty link_row field value in a publicly form"

What is in this MR

This merge request fixes a bug where it was not possible to submit a form with containing a link_row field with an empty value.

How to test this MR

• Create a table with a link row field, a form view, and add the link row field to the form view.
• Set the link row field as not required in the form.
  • It should be possible to submit the form without choosing any value.
  • It should be possible to submit the form when choosing the empty value.
  • It should be possible to submit the form with any value.
• Set the link row field as required in the form.
  • A required error should be shown when submitting the form without choosing any value.
  • A required error should be shown when submitting when choosing the empty value.
  • It should be possible to submit the form with any value.

Merge Request Checklist

• changelog.md has been updated if required.
• New/updated Premium/Enterprise features are separated correctly in the premium or enterprise folder
• The latest Chrome and Firefox have been used to test any new frontend features
• Documentation has been updated
• Quality Standards are met
• Performance: tables are still fast with 100k+ rows, 100+ field tables
• The redoc API pages have been updated for any REST API changes
• Our custom API docs are updated for changes to endpoints accessed via api tokens
• The UI/UX has been updated following UI Style Guide

Closes #1914 (closed)