$wp_filesystem->dirlist() can return false and that should be checked for before iterating over the return value

[pbiron]

There are a number of cases in core (e.g., https://core.trac.wordpress.org/browser/trunk/src/wp-admin/includes/class-wp-upgrader.php#L500) where the return value of $wp_filesystem->dirlist() is assumed to be an array. But since that method can return false, we should always check for that before iterating (or calling array_keys() or array_values() on it) over the return value.

Trac ticket: https://core.trac.wordpress.org/ticket/55387

[costdev]

As this ticket still needs more work to investigate and implement an appropriate solution to each instance, I'm moving this to Future Release.