A better fix for #55817 (thanks to bjori)

Author: laruence

ext/mbstring/mbstring.c

@@ -3183,7 +3183,7 @@ PHP_FUNCTION(mb_detect_encoding)
 	/* make encoding list */
 	list = NULL;
 	size = 0;
-	if (ZEND_NUM_ARGS() >= 2) {
+	if (ZEND_NUM_ARGS() >= 2 && !ZVAL_IS_NULL(encoding_list)) {
 		switch (Z_TYPE_P(encoding_list)) {
 		case IS_ARRAY:
 			if (FAILURE == php_mb_parse_encoding_array(encoding_list, &list, &size, 0 TSRMLS_CC)) {