Don't disallow dropping NOT NULL for a list partition key.

Range partitioning doesn't support nulls in the partitioning columns,
but list partitioning does.

Amit Langote, per a complaint from Amul Sul

src/backend/commands/tablecmds.c

@@ -5593,6 +5593,9 @@ ATExecDropNotNull(Relation rel, const char *colName, LOCKMODE lockmode)
 	if (rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
 	{
 		PartitionKey key = RelationGetPartitionKey(rel);
+
+		if (get_partition_strategy(key) == PARTITION_STRATEGY_RANGE)
+		{
 			int			partnatts = get_partition_natts(key),
 						i;
 
@@ -5607,6 +5610,7 @@ ATExecDropNotNull(Relation rel, const char *colName, LOCKMODE lockmode)
 									colName)));
 			}
 		}
+	}
 
 	/*
 	 * Okay, actually perform the catalog change ... if needed

src/test/regress/expected/alter_table.out

@@ -3029,6 +3029,10 @@ ERROR:  cannot alter type of column referenced in partition key expression
 -- cannot drop NOT NULL on columns in the range partition key
 ALTER TABLE partitioned ALTER COLUMN a DROP NOT NULL;
 ERROR:  column "a" is in range partition key
+-- it's fine however to drop one on the list partition key column
+CREATE TABLE list_partitioned (a int not null) partition by list (a);
+ALTER TABLE list_partitioned ALTER a DROP NOT NULL;
+DROP TABLE list_partitioned;
 -- partitioned table cannot participate in regular inheritance
 CREATE TABLE foo (
 	a int,

src/test/regress/sql/alter_table.sql

@@ -1917,6 +1917,11 @@ ALTER TABLE partitioned ALTER COLUMN b TYPE char(5);
 -- cannot drop NOT NULL on columns in the range partition key
 ALTER TABLE partitioned ALTER COLUMN a DROP NOT NULL;
 
+-- it's fine however to drop one on the list partition key column
+CREATE TABLE list_partitioned (a int not null) partition by list (a);
+ALTER TABLE list_partitioned ALTER a DROP NOT NULL;
+DROP TABLE list_partitioned;
+
 -- partitioned table cannot participate in regular inheritance
 CREATE TABLE foo (
 	a int,