Changed the default value for the "allow_blank" parameter to None

While developing with this helpful library, I noticed that when editing a document with validation that the XML attribute allow_blank was dropped from the data validation object. After some debugging, I found out, that the default value of the allow_blank parameter in the constructor of the DataValidation class overrode the actual value passed to the allowBlank parameter.

Probably further testing would have prevented this bug from being implemented in !419 (merged), but for now I have simply corrected the default value for the allow_blank parameter to None, which should fix the bug and not change the idea behind the default values.