Make entirely-dummy appendrels get marked as such in set_append_rel_size.

The planner generally expects that the estimated rowcount of any relation
is at least one row, *unless* it has been proven empty by constraint
exclusion or similar mechanisms, which is marked by installing a dummy path
as the rel's cheapest path (cf. IS_DUMMY_REL).  When I split up
allpaths.c's processing of base rels into separate set_base_rel_sizes and
set_base_rel_pathlists steps, the intention was that dummy rels would get
marked as such during the "set size" step; this is what justifies an Assert
in indxpath.c's get_loop_count that other relations should either be dummy
or have positive rowcount.  Unfortunately I didn't get that quite right
for append relations: if all the child rels have been proven empty then
set_append_rel_size would come up with a rowcount of zero, which is
correct, but it didn't then do set_dummy_rel_pathlist.  (We would have
ended up with the right state after set_append_rel_pathlist, but that's
too late, if we generate indexpaths for some other rel first.)

In addition to fixing the actual bug, I installed an Assert enforcing this
convention in set_rel_size; that then allows simplification of a couple
of now-redundant tests for zero rowcount in set_append_rel_size.

Also, to cover the possibility that third-party FDWs have been careless
about not returning a zero rowcount estimate, apply clamp_row_est to
whatever an FDW comes up with as the rows estimate.

Per report from Andreas Seltenreich.  Back-patch to 9.2.  Earlier branches
did not have the separation between set_base_rel_sizes and
set_base_rel_pathlists steps, so there was no intermediate state where an
appendrel would have had inconsistent rowcount and pathlist.  It's possible
that adding the Assert to set_rel_size would be a good idea in older
branches too; but since they're not under development any more, it's likely
not worth the trouble.

src/test/regress/expected/join.out

@@ -2164,6 +2164,26 @@ select count(*) from tenk1 x where
 (1 row)
 
 rollback;
+--
+-- regression test: be sure we cope with proven-dummy append rels
+--
+explain (costs off)
+select aa, bb, unique1, unique1
+  from tenk1 right join b on aa = unique1
+  where bb < bb and bb is null;
+        QUERY PLAN        
+--------------------------
+ Result
+   One-Time Filter: false
+(2 rows)
+
+select aa, bb, unique1, unique1
+  from tenk1 right join b on aa = unique1
+  where bb < bb and bb is null;
+ aa | bb | unique1 | unique1 
+----+----+---------+---------
+(0 rows)
+
 --
 -- Clean up
 --

src/test/regress/sql/join.sql

@@ -353,6 +353,17 @@ select count(*) from tenk1 x where
   x.unique1 in (select aa.f1 from int4_tbl aa,float8_tbl bb where aa.f1=bb.f1);
 rollback;
 
+--
+-- regression test: be sure we cope with proven-dummy append rels
+--
+explain (costs off)
+select aa, bb, unique1, unique1
+  from tenk1 right join b on aa = unique1
+  where bb < bb and bb is null;
+
+select aa, bb, unique1, unique1
+  from tenk1 right join b on aa = unique1
+  where bb < bb and bb is null;
 
 --
 -- Clean up
@@ -1120,6 +1131,7 @@ select atts.relid::regclass, s.* from pg_stats s join
     a.attrelid::regclass::text join (select unnest(indkey) attnum,
     indexrelid from pg_index i) atts on atts.attnum = a.attnum where
     schemaname != 'pg_catalog';
+
 --
 -- Test LATERAL
 --