Use coalesce to use column_full_type when column_type is null for user defined types

Fix Scan error on column index 1, name "column_type": converting NULL to string is unsupported, by coalescing column_type and if null replacing with column_full_type value.

Author: mattdbush

drivers/sqlboiler-psql/driver/psql.go

@@ -567,7 +567,7 @@ func (p *PostgresDriver) Columns(schema, tableName string, whitelist, blacklist
 
 	query := fmt.Sprintf(`SELECT 
 		column_name,
-		column_type,
+		COALESCE(column_type, column_full_type) as column_type,
 		column_full_type,
 		udt_name,
 		array_type,