Not setting #weight in hook_domain_batch causes issues with hook_menu

I've just implemented hook_domain_batch in a custom module and now I have issues creating Views page displays. They create fine but don't add an entry to the menu_router table and therefore are not viewable. After 2 hours or so of debugging I found that weight is not actually "optional" as per the documentation in domain.api.php.

I've resolved this in the attached patch by defaulting the weight to 0 in domain_menu() if it's not set.