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 4.8 branch.


Built from https://develop.svn.wordpress.org/branches/4.8@41484


git-svn-id: http://core.svn.wordpress.org/branches/4.8@41317 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.8.2-alpha-41471';
+$wp_version = '4.8.2-alpha-41484';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.

wp-includes/wp-db.php

@@ -1306,7 +1306,7 @@ 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 ) ), '4.8.2' );
 			}
 		}