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.
@garoevans I can live with the change but your use case seems to be dealing with another issue. You want to make sure no useless alter table statements are created but even without this patch, a mapping that does not explicitly define a default should not trigger alter table statements if the database schema als uses `null` as default. This sounds more like an issue with the DBAL comparator then, rather than a mapping issue.
To um up: It should not be required to define `options={"default":NULL}` just to come around useless alter table statements, get what I mean?