What is a null CDT ?How do I check whether the CDT is null ?

A null CDT would be a data structure that does not have any values stored in it for your specific instance. So for example you have a cdt of type employee, and it looked like this - 

CDT  - Employee
Fields - empID, firstName, lastName

A populated CDT would look like - 
type!{namespace}Employee(empID: 1, firstName :"ABC", lastName:"XYZ" )

And a null CDT would be one without any values with just an empty structure - where the value would show as 'null'.
Another level to look at it is where the CDT exists but having empty fields - 'type!{namespace}Employee(), this creates a CDT with fields that have empty values - type!{namespace}Employee(empID: null, firstName :"", lastName:"" )

You will notice this when you have an ri of a CDT on an interface where the CDT has not been populated with values yet. Once it gets populated the ri can be split to see the values for each field.

A null CDT would be a data structure that does not have any values stored in it for your specific instance. So for example you have a cdt of type employee, and it looked like this - 

CDT  - Employee
Fields - empID, firstName, lastName

A populated CDT would look like - 
type!{namespace}Employee(empID: 1, firstName :"ABC", lastName:"XYZ" )

And a null CDT would be one without any values with just an empty structure - where the value would show as 'null'.
Another level to look at it is where the CDT exists but having empty fields - 'type!{namespace}Employee(), this creates a CDT with fields that have empty values - type!{namespace}Employee(empID: null, firstName :"", lastName:"" )

You will notice this when you have an ri of a CDT on an interface where the CDT has not been populated with values yet. Once it gets populated the ri can be split to see the values for each field.