If the ::single_value_transformer() find an existing HAVING condition it used
to do the transformation:
            
  1) HAVING cond -> (HAVING Cond) AND (cond_guard (Item_ref_null_helper(...))
      
As the AND condition in 1) is Mc'Carty evaluated, the
right side of the AND cond should be executed only if the 
original 'HAVING evaluated' to true.
      
However, as we failed to set 'top_level' for the tranformed HAVING condition,
'abort_on_null' was FALSE after transformation. An
UNKNOWN having condition will then not terminate evaluation of the
transformed having condition, and we incorrectly continued
into the Item_ref_null_helper() part.