Prevent a notice: don't attempt to parse the query string if it doesn't exist....

Prevent a notice: don't attempt to parse the query string if it doesn't exist. Props johnjamesjacoby. Fixes #20528.


git-svn-id: https://core.svn.wordpress.org/trunk@20669 1a063a9b-81f0-0310-95a4-ce76da25c4cd