Added ability to check if expandMdm is null for bullk export

Author: rob-caruso

hapi-fhir-docs/src/main/resources/ca/uhn/hapi/fhir/changelog/8_8_0/7479-tri-state-logic-for-mdm-expansion.yaml

@@ -0,0 +1,8 @@
+---
+type: fix
+issue: 7479
+title: "The expandMdm field in bulk export operations now properly supports 
+  tri-state logic to distinguish between explicitly disabled (false), 
+  explicitly enabled (true), and not specified (null). This allows the system 
+  to apply appropriate default behavior when the _mdm parameter set explicitly 
+  to false as opposed to being omitted"