Omit empty optional fields

[Pajk]

Resolves #ISSUE_NUMBER

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Before the change?

• Sending empty strings for optional fields domain (header, cookie) and path (cookie).

After the change?

• Omit the field if the value is empty string.

Pull request checklist

• Acceptance Tests have been updated, run (make testacc), and pasted in this PR (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Acceptance Test Output

Does this introduce a breaking change?

• Yes
• No

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