theme.json file variable could be a directory, add check to prevent notices

[ramon fincken]

In class-wp-theme-json-resolver.php

​https://github.com/WordPress/wordpress-develop/blob/6.8.1/src/wp-includes/class-wp-theme-json-resolver.php#L254

is the line

if ( is_readable( $theme_json_file ) ) {

However if the $theme_json_file is a directory that will return true, even if it expects a .json local file path.

In this patch I propose adding is_file ( and perhaps even removing is_readable, as is_readable will also return true for directories ).

In this patch I propose adding is_file ( and perhaps even removing is_readable, as is_readable will also return true for directories ).

Trac
https://core.trac.wordpress.org/ticket/63572#ticket