Make sure mailerTransport can be NULL

[aschempp]

While updating from Contao 4.13 and Notification Center 1.*, I got the following error because the field appears to be non-nullable in version 1

SQLSTATE[23000]: Integrity constraint violation: 1048 Column 'mailerTransport' cannot be null

[fritzmg]

Alternatively you could check for

if ($columns['mailertransport']->getNotnull()) {
    return false;
}

in shouldRun(), then the migration will only be executed after the field has been made nullable by the regular database updates.

[fritzmg]

Suggested change

	$this->connection->executeStatement("ALTER TABLE tl_nc_gateway CHANGE mailerTransport mailterTransport varchar(64) DEFAULT NULL");
	$this->connection->executeStatement("ALTER TABLE tl_nc_gateway CHANGE mailerTransport mailerTransport varchar(64) DEFAULT NULL");