get_preferred_from_update_core can return false

[markoheijnen]

I just had a case in my network installation where get_site_transient( 'update_core' ) returned false.
In this case it gave an error message at core_update_footer();

When I went back to Network admin updates page and press 'check_again' the error went away.
The error can be caused because of running the recent nightly build (3.4-alpha-20205).

What was weird that in the site wp-admin there still was the balloon that there was an update when I had the error. When I pressed the button the balloon went away what is the correct.