Fix the issue that HTTP request body contains empty userAssignedIdentities object when identity type is SystemAssigned (#26941)

Author: wyunchi-ms

src/Aks/Aks/ChangeLog.md

@@ -18,6 +18,7 @@
         - Additional information about change #1
 -->
 ## Upcoming Release
+* Fixed the issue that HTTP request body contains empty userAssignedIdentities object when identity type is `SystemAssigned`.
 
 ## Version 6.0.4
 * Fixed secrets exposure in example documentation.

src/Aks/Aks/Commands/CreateOrUpdateKubeBase.cs

@@ -698,6 +698,7 @@ protected ManagedCluster SetIdentity(ManagedCluster cluster)
                 if (cluster.Identity != null && cluster.Identity.Type == null)
                 {
                     cluster.Identity.Type = ResourceIdentityType.SystemAssigned;
+                    cluster.Identity.UserAssignedIdentities = null;
                 }
             }