- calling expr.in_([]), i.e. with an empty list, emits a warning

- calling expr.in_([]), i.e. with an empty list, emits a warning
before issuing the usual "expr != expr" clause.  The
"expr != expr" can be very expensive, and it's preferred
that the user not issue in_() if the list is empty,
instead simply not querying, or modifying the criterion
as appropriate for more complex situations.
[ticket:1628]