to NULL

For queries of the form SELECT MIN(key_part_k) FROM t1 
WHERE key_part_1 = const and ... and key_part_k-1 = const,
the opt_sum_query optimization tries to
use an index to substitute MIN/MAX functions with their values according
to the following rules:
1) Insert the minimum non-null values where the WHERE clause still matches, or
3) A row of nulls

However, the correct semantics requires that there is a third case 2)
such that a NULL value is substituted if there are only NULL values for 
key_part_k.

The patch modifies opt_sum_query() to handle this missing case.


mysql-test/r/func_group.result:
  Bug #27573: Correct result
mysql-test/t/func_group.test:
  Bug #27573: test case
sql/opt_sum.cc:
  Bug #27573:
  Added code that will try to read the
  first non-null value for a given complete-field prefix, second
  choice is to read the null, and lastly set the error code if no row
  is found.