reader/rss: don't add empty tags to RSS items

[jvoisin]

This commit adds a bunch of checks to prevent reader/rss from adding empty tags to rss items, as well as some minor refactors like nested conditions and loops unrolling.

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• I have tested my changes
• There is no breaking changes
• I really tested my changes and there is no regression
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[jvoisin]

This is part of #2563