Avoid using list_length() to test for empty list.

The standard way to check for list emptiness is to compare the
List pointer to NIL; our list code goes out of its way to ensure
that that is the only representation of an empty list.  (An
acceptable alternative is a plain boolean test for non-null
pointer, but explicit mention of NIL is usually preferable.)

Various places didn't get that memo and expressed the condition
with list_length(), which might not be so bad except that there
were such a variety of ways to check it exactly: equal to zero,
less than or equal to zero, less than one, yadda yadda.  In the
name of code readability, let's standardize all those spellings
as "list == NIL" or "list != NIL".  (There's probably some
microscopic efficiency gain too, though few of these look to be
at all performance-critical.)

A very small number of cases were left as-is because they seemed
more consistent with other adjacent list_length tests that way.

Peter Smith, with bikeshedding from a number of us

Discussion: https://postgr.es/m/CAHut+PtQYe+ENX5KrONMfugf0q6NHg4hR5dAhqEXEc2eefFeig@mail.gmail.com

src/backend/optimizer/plan/createplan.c

@@ -2462,7 +2462,7 @@ create_groupingsets_plan(PlannerInfo *root, GroupingSetsPath *best_path)
 
 			if (rollup->is_hashed)
 				strat = AGG_HASHED;
-			else if (list_length(linitial(rollup->gsets)) == 0)
+			else if (linitial(rollup->gsets) == NIL)
 				strat = AGG_PLAIN;
 			else
 				strat = AGG_SORTED;