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.
Also please not [this line](https://github.com/doctrine/dbal/blob/master/lib/Doctrine/DBAL/Platforms/AbstractPlatform.php#L2247). That makes it even more confusing why your generated SQL does not contain the `DEFAULT NULL` clause as you mapped a nullable column.