List Tables: Fix PHP error notice when $columns is null

Use of `register_column_headers()` and `print_column_headers()` creates a `_WP_List_Table_Compat` without any columns. When the List Table object doesn't have any columns, there's naturally no primary column.

Props danielbachhuber.
Fixes #34148.

Built from https://develop.svn.wordpress.org/trunk@35698


git-svn-id: http://core.svn.wordpress.org/trunk@35662 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.4-beta4-35697';
+$wp_version = '4.4-beta4-35698';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.