Make sure that Query_result_null (used during EXPLAIN ANALYZE) evaluates all
the arguments in the SELECT list, even though it doesn't actually need its
results for anything. This is so that any embedded subqueries will be executed
and get proper row counts and timings.

There are other potentially expensive items that we don't show timing
information for; imagine e.g. an UDF that takes a lot of time, or even SLEEP().
(Evaluating normal items generally don't take a lot of time.) However, these
are fairly rare, so we will ignore them for now.

Backport to 8.0.

Change-Id: Ia04629a7561af7529fcfc4df74e05a5cd1f3ae40