Add null clause test cases to router planner regression tests

Router planner already handles cases when all shards
are pruned out. This is about missing test cases. Notice that
"column is null" and "column = null" have different shard
pruning behavior.

src/test/regress/expected/multi_router_planner.out

@@ -1385,6 +1385,26 @@ DEBUG:  Plan is router executable
 --------------+-------------------
 (0 rows)
 
+-- partition_column is null clause does not prune out any shards,
+-- all shards remain after shard pruning, not router plannable
+SELECT * 
+	FROM articles_hash a
+	WHERE a.author_id is null;
+ id | author_id | title | word_count 
+----+-----------+-------+------------
+(0 rows)
+
+-- partition_column equals to null clause prunes out all shards
+-- no shards after shard pruning, router plannable
+SELECT * 
+	FROM articles_hash a
+	WHERE a.author_id = null;
+DEBUG:  Creating router plan
+DEBUG:  Plan is router executable
+ id | author_id | title | word_count 
+----+-----------+-------+------------
+(0 rows)
+
 -- stable function returning bool
 SELECT * 
 	FROM articles_hash a
@@ -1595,7 +1615,7 @@ WARNING:  relation "public.articles_range" does not exist
 CONTEXT:  while executing command on localhost:57637
 WARNING:  relation "public.articles_range" does not exist
 CONTEXT:  while executing command on localhost:57637
-ERROR:  failed to execute job 840020
+ERROR:  failed to execute job 840021
 DETAIL:  Failure due to failed task 2
 -- same query with where false but evaluation left to worker
 SELECT author_id FROM articles_range
@@ -1611,7 +1631,7 @@ WARNING:  relation "public.articles_range" does not exist
 CONTEXT:  while executing command on localhost:57637
 WARNING:  relation "public.articles_range" does not exist
 CONTEXT:  while executing command on localhost:57637
-ERROR:  failed to execute job 840021
+ERROR:  failed to execute job 840022
 DETAIL:  Failure due to failed task 2
 -- same query on router planner with where false but evaluation left to worker
 SELECT author_id FROM articles_single_shard_hash

src/test/regress/sql/multi_router_planner.sql

@@ -580,6 +580,18 @@ SELECT a.author_id as first_author, b.word_count as second_word_count
 	FROM articles_hash a, articles_single_shard_hash b
 	WHERE a.author_id = 10 and a.author_id = b.author_id and int4eq(1, 2);
 
+-- partition_column is null clause does not prune out any shards,
+-- all shards remain after shard pruning, not router plannable
+SELECT * 
+	FROM articles_hash a
+	WHERE a.author_id is null;
+
+-- partition_column equals to null clause prunes out all shards
+-- no shards after shard pruning, router plannable
+SELECT * 
+	FROM articles_hash a
+	WHERE a.author_id = null;
+
 -- stable function returning bool
 SELECT * 
 	FROM articles_hash a