selftest/s4: properly initialise an empty hash

selftest/s4: properly initialise an empty hash

The '%ret = {}' construction was bad because '{}' is a hash-ref, which
counts as a single scalar value, but a true hash like '%ret' must be
initialised with an even number of scalar values (usually in pairs, like
'($a => $b, $c => $d)').

I think this meant %ret was initialised as something harmless like
'(<HASH(0x55ce39781278)> => undef)'.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>