Simplify PathKey checking code

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