Populate playlist setting values with their default value if they don't exist while parsing shortcode attributes.

Props kovshenin.
See #26631.


Built from https://develop.svn.wordpress.org/trunk@27308


git-svn-id: http://core.svn.wordpress.org/trunk@27161 1a063a9b-81f0-0310-95a4-ce76da25c4cd