Bail early from shortcode functions if no delimiter is present.

This is a significant performance improvement for processing content without shortcodes, and only the slightest hit when content contains shortcodes (which must then undergo processing anyway). Performance results on the ticket.

props TobiasBg.
fixes #23855.


git-svn-id: https://develop.svn.wordpress.org/trunk@27394 602fd350-edb4-49c9-b593-d223f7449a82