fetchxml does not reuturn column in dynamics

I am using Python to send a http GET request to extract the contacts associated with a segment. The segment for example called "Test Segment" which has a unique identifier "a1-b2-c3".

Below is a fetchXML query I am using:

<fetch mapping="logical" output-format="xml-platform" version="1.0" page="1" count="5000" paging-cookie=""> 
  <entity name="contact"> 
  <attribute name="fullname"/> 
  <attribute name="contactid"/> 
  <attribute name="emailaddress1"/> 
  <attribute name = "btp_contacttypeid"/>
  <order attribute="fullname" descending="false"/> 
  <link-entity name="msdyncrm_segment" from="msdyncrm_segmentid" to="msdyncrm_segmentmemberid" alias="bb"> 
  <attribute name="msdyncrm_segmentname" />
  <filter type="and"> 
  <condition attribute="msdyncrm_segmentid" operator="eq" uitype="msdyncrm_segment" value="a1-b2-c3"/>
  </filter>
  </link-entity>
  </entity>
  </fetch>

I get the following output:

I believe fetchXML automatically removes the null values columns. Technically it is not null but not sure why it returns null Expected Output:

Can you please help with the following problem?

This problem occurs in two diffrent scenario

1. Permissions to field
2. Null Value

When you execute query over dynamics crm sdk, attribute with null value does not exist in entity. Same problem occurs when user does not have a permission to field.