[cells] distiguish null and undefined in cell's default equality

Otherwise transition from null to undefined or vice versa will not be
signalled to subscribers. In a number of places where we use null
we're defining special equality that deals with nulls. But with this
change there's no need for that anymore.

Change-Id: I94ec1d14b865730ebb015e8b07a44637e54284fa
Reviewed-on: http://review.couchbase.org/39964
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Pavel Blagodov <stochmail@gmail.com>
Tested-by: Pavel Blagodov <stochmail@gmail.com>