[Migrations] Drop not null is not working in Postgres

When removing the not null from a column the migration does not change anything in the database. This is due to the following check on line 162 of lib/vendor/symfony/lib/plugins/sfDoctrinePlugin/lib/vendor/doctrine/Doctrine/Export/Pgsql.php
if ( ! empty($field['definition']['notnull']))

So if notnull is not there or set to false or '0' or 0 the code does not enter into that if statement and therefore no changes are done to the not null value of the column.