I have used null-able data types as data contract in WCF and its working in live environment for last 2.5 years.
Actual problem at your WSDL your code generator unable to understand class or property details from it.
How to Find wsdl : http://[Your Domain or IP/[Folder]/[Service File Name]?wsdl
When you consume a service at your application (.Net) it will call its wsdl and create the proxy class, now what you are using it will work WCF to .Net application but what about other language.
Now consume it as web reference it will solve your problem.
How to Add Web reference : http://msdn.microsoft.com/en-in/library/ms169816.aspx
Service Reference is .Net to .Net and Web reference .Net to other language.
There are some other work around also like modifying your WSDL : http://stackoverflow.com/questions/2426892/wcf-datacontract-does-it-support-nullable-data-member
Or Generate Proxy class at your own and see what error is there : http://msdn.microsoft.com/en-IN/library/ms155134.aspx