Database: Don’t trigger `_doing_it_wrong()` for null values in `wpdb::prepare()`.

While `wpdb::prepare()` does not support null values (see #12819) they still appear in the wild like in the WordPress Importer and other plugins.

Merges [41483] to 3.9 branch.


Built from https://develop.svn.wordpress.org/branches/3.9@41493


git-svn-id: http://core.svn.wordpress.org/branches/3.9@41326 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/wp-db.php

@@ -1207,7 +1207,7 @@ class wpdb {
 		}
 
 		foreach ( $args as $arg ) {
-			if ( ! is_scalar( $arg ) ) {
+			if ( ! is_scalar( $arg ) && ! is_null( $arg ) ) {
 				_doing_it_wrong( 'wpdb::prepare', sprintf( 'Unsupported value type (%s).', gettype( $arg ) ), '3.9.20' );
 			}
 		}