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.
For my specific use case, trying to come over to doctrine using an existing schema. The productions databases are fairly large and not planning on being changed any time soon. To use the schema tool the entities need to be handle a specifically declared null default to not force an alter statement regardless of change.

Also, regardless of the use case, this is "correct" where as the code previously had a bug, I think that is fair to say?