Set user_defined_types.data_type to null

On re-reading the standard, this field is only used for distinct or
reference types.

doc/src/sgml/information_schema.sgml

@@ -5925,11 +5925,7 @@ ORDER BY c.ordinal_position;
      <row>
       <entry><literal>data_type</literal></entry>
       <entry><type>character_data</type></entry>
-      <entry>
-       Always <literal>USER-DEFINED TYPE</literal> (for joining
-       against <literal>object_type</literal> columns in other
-       views)
-      </entry>
+      <entry>Applies to a feature not available in <productname>PostgreSQL</></entry>
      </row>
 
      <row>

src/backend/catalog/information_schema.sql

@@ -2198,7 +2198,7 @@ CREATE VIEW user_defined_types AS
            CAST(null AS sql_identifier) AS ordering_routine_schema,
            CAST(null AS sql_identifier) AS ordering_routine_name,
            CAST(null AS character_data) AS reference_type,
-           CAST('USER-DEFINED TYPE' AS character_data) AS data_type,
+           CAST(null AS character_data) AS data_type,
            CAST(null AS cardinal_number) AS character_maximum_length,
            CAST(null AS cardinal_number) AS character_octet_length,
            CAST(null AS sql_identifier) AS character_set_catalog,