When removing the max-weightsystem restriction, the semantics of the DiveObjectHelper::singleWeightSystem() function changed: it now returned false for "no weightsystem". Change it back, to 0 or 1 weightsystems, because the mobile frontend uses this to check whether it can edit dive systems. Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>