Fix PHP notice in `Tests_Feed_RSS2::test_items()`.

A notice was being thrown when `get_the_tags()` returned false, and `foreach()`
was run over that return value.

See [30283].

git-svn-id: https://develop.svn.wordpress.org/trunk@30295 602fd350-edb4-49c9-b593-d223f7449a82