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].
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git-svn-id: http://core.svn.wordpress.org/trunk@30294 1a063a9b-81f0-0310-95a4-ce76da25c4cd