active record find conditions currently don't work correctly if an array with a nil in it is used

I've created a patch in lighthouse for this and looking for a bit of
feedback.

Essential the problem is due to MySQL (and perhaps other database
vendors) of returning an empty result set for:

select * from somewhere where name in (null)

regardless of whether there are records with null entries in that
field. the version that does work is:

select * from somewhere where name is null

with active record this means that the following does not return the
expected records:

Event.find(:all, :conditions => {:venue_id => [1,2,nil]})

http://rails.lighthouseapp.com/projects/8994/tickets/1486-active-record-conditions-to-deal-with-nils-in-arrays

cheers,
n