Fix 500 errors on searching invalid URLs

[unarist]

Fixes #3461

Searching invalid feed URLs causes 500 error from #2812. It should returns empty results without error.

For example:

• not contains feed URL: http://example.com/
• non-200 responses: https://mstdn.maud.io/@unarist/2346775 (you can't see because it's a "direct" toot)

Actually, FetchAtomService returns nil when the specified URL returns non-200 responses or when it doesn't have any atom links, not [nil, ...].

cc @mjankowski