Fix print_error() return type: void|false → null|false

Add explicit return null at the end of the method to match the
documented return type.

Co-Authored-By: Claude Opus 4.6 (1M context) <[email protected]>

Author: westonruter

src/wp-includes/class-wpdb.php

@@ -1794,7 +1794,7 @@ public function esc_like( $text ) {
 	 * @global array $EZSQL_ERROR Stores error information of query and error string.
 	 *
 	 * @param string $str The error to display.
-	 * @return void|false Void if the showing of errors is enabled, false if disabled.
+	 * @return null|false Null if the showing of errors is enabled, false if disabled.
 	 */
 	public function print_error( $str = '' ) {
 		global $EZSQL_ERROR;
@@ -1855,6 +1855,8 @@ public function print_error( $str = '' ) {
 				$query
 			);
 		}
+
+		return null;
 	}
 
 	/**