Problem: Subqueries with implicit grouping may return incorrect
results due to not evaluating the WHERE clause.

Queries with ALL/ANY are rewritten to use MIN/MAX. The rewritten form
of the query has no GROUP BY clause, so these queries fit the pattern
for triggering this bug.

JOIN::optimize() calls opt_sum_query() to replace aggregation
functions with constant fields for implicitly grouped queries. If the
aggregation function is MIN or MAX, it checks the argument to the
aggregation function. If it is a constant, the constant is selected as
the minimum/maximum value if the table has rows, and NULL if it
doesn't. However, the WHERE clause is not checked, so the MIN/MAX may
be evaluated to a constant when it should have been NULL.

Fix: Don't replace MIN/MAX of constants if there is a WHERE clause.