NULL from outer join query
      
      Problem: optimising MIN/MAX() queries without GROUP BY clause
      by replacing the aggregate expression with a constant, we may set it
      to NULL disregarding the fact that there may be outer joins involved.
      
      Fix: don't replace MIN/MAX() with NULL if there're outer joins.
      
      Note: the fix itself is just
      - if (!count)
      + if (!count && !outer_tables)
          set to NULL
      
      The rest of the patch eliminates repeated code to improve speed
      and for easy maintenance of the code.