Don't generate a query when the all_page_ids cache is an empty array. An empty...

Don't generate a query when the all_page_ids cache is an empty array. An empty array is a valid cache value when no pages are present. Avoids a useless query for sites with no pages. Props andy. see #20236

git-svn-id: https://core.svn.wordpress.org/trunk@20216 1a063a9b-81f0-0310-95a4-ce76da25c4cd