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.


mysql-test/r/group_by.result:
          Fix for bug#52051: Aggregate functions incorrectly returns
          NULL from outer join query
            - test result.
mysql-test/t/group_by.test:
          Fix for bug#52051: Aggregate functions incorrectly returns
          NULL from outer join query
            - test case.
sql/opt_sum.cc:
          Fix for bug#52051: Aggregate functions incorrectly returns
          NULL from outer join query
            - optimising MIN/MAX() queries without GROUP BY clause by
          replacing them with a constant, take into account that
          there're may be outer joins involved.
            - repeated code for MIN/MAX optimization in the opt_sum_query()
          eliminated by introducing new functions that read MIN/MAX values
          using index and combining MIN/MAX cases to one.