_changes shouldn't fail if user has no channels
_changes shouldn't fail if user has no channels

If _changes is called with no explicit channel list, and the user has
access to no channels, it shouldn't return an error. (Instead it's just
a no-op.)

tophatch/sync_gateway,tleyden/sync_gateway,paulharter/sync_gateway,evankyle/sync_gateway,couchbaselabs/sync_gateway,paulharter/sync_gateway,ceejatec/sync_gateway,paulharter/sync_gateway,tleyden/sync_gateway,couchbaselabs/sync_gateway,evankyle/sync_gateway,tophatch/sync_gateway,jamiltz/sync_gateway,paulharter/sync_gateway,tleyden/sync_gateway,ceejatec/sync_gateway,tophatch/sync_gateway,evankyle/sync_gateway,ceejatec/sync_gateway,evankyle/sync_gateway,tophatch/sync_gateway,couchbaselabs/sync_gateway,jamiltz/sync_gateway,ceejatec/sync_gateway,couchbaselabs/sync_gateway,tleyden/sync_gateway,jamiltz/sync_gateway