Fix multiple categories and backward compatibility

[Soare-Robert-Daniel]

Summary

The separator for saved categories was \r\n and not just \n. Regression introduced in 73ea813#diff-de5716d7b637a0feed0d4f345ffb3971f14f694081d2af8dfc59a4889d152155R596

Changes:

• Fixed the separator to be \r\n on saving.
• The parser will work both on \n and \r\n.
• Do not save tags if they are not present in the request.

Will affect visual aspect of the product

NO

Test instructions

• Create a group field
• Attach it to some categories and check if it works.

Check before Pull Request is ready:

• I have written a test and included it in this PR
• I have run all tests and they pass
• The code passes when running the PHP CodeSniffer
• Code meets WordPress Coding Standards for PHP, HTML, CSS and JS
• Security and Sanitization requirements have been followed
• I have assigned a reviewer or two to review this PR (if you're not sure who to assign, we can do this step for you)

Closes https://github.com/Codeinwp/ppom-pro/issues/496

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🎉 This PR is included in version 33.0.7 🎉

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