Checks key exists rather than isset

If the default value is set to `null`, `isset` will return `false` even though the key is actually there for a reason.
Okay I'll try to explain a bit more in detail.
If you instantiate a `Doctrine\DBAL\Schema\Column` object like this

```php
$type = \Doctrine\DBAL\Types\Type::getType('string');

$column = new \Doctrine\DBAL\Schema\Column('some_field', $type);
$column->getDefault(); // NULL

$column = new \Doctrine\DBAL\Schema\Column('some_field', $type, array('default' => null));
$column->getDefault(); // NULL

// see: https://github.com/doctrine/dbal/blob/master/lib/Doctrine/DBAL/Schema/Column.php#L71
```
both columns are equal because `\Doctrine\DBAL\Schema\Column::$default` is `null` by default. So when the ORM schema tool builds columns from your mappings, it does not matter whether you define `options={"default":NULL}` or leave it.
When the existing schema is introspected by Doctrine DBAL, a column with a null default will be created equally to the above. Therefore the comparator should not detect schema changes on `doctrine:schema:update` and therefore should not ouput any SQL. But in your case it does so I assume there is a problem in DBAL with the comparator.
Can you maybe check your use case using DBAL 2.5 as I think to remember that we have fixed some things about default value comparison in that branch.