ER Diagram-How to add NULL/NOT NULL to the DDL?

As workaround you can change query and manually add NOT_NULL to the data_type using concat function

for postgresql:

SET enable_nestloop = 0;
SELECT 'postgresql' AS dbms
       t.table_catalog
       t.table_schema
       t.table_name
       c.column_name
       c.ordinal_position
       concat(c.data_type ' ' replace(replace(k3.is_nullable 'YES' '') 'NO' 'NOT_NULL')) as "c.data_type"
       c.character_maximum_length
       n.constraint_type
       k2.table_schema
       k2.table_name
       k2.column_name
FROM information_schema.tables t
         NATURAL LEFT JOIN information_schema.columns c
         LEFT JOIN(information_schema.key_column_usage k NATURAL JOIN information_schema.table_constraints n NATURAL LEFT JOIN information_schema.referential_constraints r)
                  ON c.table_catalog = k.table_catalog AND c.table_schema = k.table_schema AND
                     c.table_name = k.table_name AND c.column_name = k.column_name
         LEFT JOIN information_schema.key_column_usage k2 ON k.position_in_unique_constraint = k2.ordinal_position AND
                                                             r.unique_constraint_catalog = k2.constraint_catalog AND
                                                             r.unique_constraint_schema = k2.constraint_schema AND
                                                             r.unique_constraint_name = k2.constraint_name
        INNER JOIN information_schema.columns k3 ON k3.column_name = c.column_name AND
                                                    k3.table_name = c.table_name
WHERE t.TABLE_TYPE = 'BASE TABLE'
  AND t.table_schema NOT IN ('information_schema' 'pg_catalog');