As per the OpenCL Extension Specification § 38.6 Ballots:
If no bits representing predicate values from all work items in
the subgroup are set in the bitfield value then the return value
is undefined.
The case with no bits set is still worth testing, as it does not result
in undefined behavior, but only an undefined return value.
Signed-off-by: Stuart Brady <stuart.brady@arm.com>