Allow the text parameter in wp_trim_excerpt() to be omitted altogether,...

Allow the text parameter in wp_trim_excerpt() to be omitted altogether, instead of requiring a blank string. props SergeyBiryukov. fixes #17492

git-svn-id: https://core.svn.wordpress.org/trunk@18824 1a063a9b-81f0-0310-95a4-ce76da25c4cd