Feeds: Ensure that enclosures produce valid XML.

Metadata that is stored on newlines has the possibility of missing values, so rather then coercing values, we can check for them and then implicity set the values.

Fixes #33591.
Props jonnybot, stevenkword, vtieu, birgire, SergeyBiryukov, davidbaumwald, rebasaurus, whyisjake.


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