Simplify PathKey checking code

pathkeys_useful_for_ordering() contained some needless checks to return
0 when either root->query_pathkeys or pathkeys lists were empty.  This is
already handled by pathkeys_count_contained_in(), so let's have it do the
work instead of having redundant checks.

Similarly, in pathkeys_useful_for_grouping(), checking pathkeys is an
empty list just before looping over it isn't required.  Technically,
neither is the list empty check for group_pathkeys, but I felt a bit
more work would have to be done to get the equivalent behavior if we'd
left it up to the foreach loop to call list_member_ptr().

This was noticed by Andy while he was reviewing a patch to improve the
UNION planner.  Since that patch adds another function similar to
pathkeys_useful_for_ordering() and since I wasn't planning to copy these
redundant checks over to the new function, let's adjust the existing
code so that both functions will be consistent.

Author: Andy Fan
Discussion: https://postgr.es/m/87o7cti48f.fsf@163.com

src/backend/optimizer/path/pathkeys.c

@@ -2143,12 +2143,6 @@ pathkeys_useful_for_ordering(PlannerInfo *root, List *pathkeys)
 {
 	int			n_common_pathkeys;
 
-	if (root->query_pathkeys == NIL)
-		return 0;				/* no special ordering requested */
-
-	if (pathkeys == NIL)
-		return 0;				/* unordered path */
-
 	(void) pathkeys_count_contained_in(root->query_pathkeys, pathkeys,
 									   &n_common_pathkeys);
 
@@ -2184,10 +2178,6 @@ pathkeys_useful_for_grouping(PlannerInfo *root, List *pathkeys)
 	if (root->group_pathkeys == NIL)
 		return 0;
 
-	/* unordered path */
-	if (pathkeys == NIL)
-		return 0;
-
 	/* walk the pathkeys and search for matching group key */
 	foreach(key, pathkeys)
 	{