Avoid a notice in trackback_rdf() if user agent is not set.

props joostdevalk.
fixes #27374.
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git-svn-id: http://core.svn.wordpress.org/trunk@27357 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Revert [27203], restore JIT color scheme stylesheets. Restores [27111].

fixes #27175. see #20729.

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git-svn-id: http://core.svn.wordpress.org/trunk@27358 1a063a9b-81f0-0310-95a4-ce76da25c4cd