Role array without 0 index preventing log from being created

[desrosj]

Working on a site and noticed debug.log instances where the following would occur:

WordPress database error Column 'user_role' cannot be null for query INSERT INTO wp_stream``

Looked around and tracked it this line in classes/class-log.php

'user_role' => (string) ! empty( $user->roles ) ? $user->roles[0] : '',

After some debugging I found that the roles property of WP_User is not guaranteed to be an array starting with an index of 0. It could be any numerical value.

Attaching a change that prevents this from blocking the log item.