XWIKI-21133: User reference serializer doesn't serialize the current user...

XWIKI-21133: User reference serializer doesn't serialize the current user reference when the current user is superadmin

* Change the code to use the same logic for the current user and an
  explicitly given user.
* Add a test.

xwiki-platform-core/xwiki-platform-user/xwiki-platform-user-default/src/main/java/org/xwiki/user/internal/document/DocumentStringUserReferenceSerializer.java

@@ -65,19 +65,21 @@ public String serialize(UserReference userReference, Object... parameters)
         String result;
         if (userReference == null) {
             result = null;
-        } else if (SuperAdminUserReference.INSTANCE == userReference) {
-            result = SUPERADMIN_REFERENCE_STRING;
-        } else if (GuestUserReference.INSTANCE == userReference) {
-            result = GUEST_REFERENCE_STRING;
-        } else if (CurrentUserReference.INSTANCE == userReference) {
-            UserReference resolvedUserReference = this.currentUserReferenceUserReferenceResolver.resolve(null);
-            if (GuestUserReference.INSTANCE == resolvedUserReference) {
+        } else {
+            UserReference resolvedReference;
+            if (CurrentUserReference.INSTANCE == userReference) {
+                resolvedReference = this.currentUserReferenceUserReferenceResolver.resolve(null);
+            } else {
+                resolvedReference = userReference;
+            }
+
+            if (SuperAdminUserReference.INSTANCE == resolvedReference) {
+                result = SUPERADMIN_REFERENCE_STRING;
+            } else if (GuestUserReference.INSTANCE == resolvedReference) {
                 result = GUEST_REFERENCE_STRING;
             } else {
-                result = serializeInternal(resolvedUserReference, parameters);
+                result = serializeInternal(resolvedReference, parameters);
             }
-        } else {
-            result = serializeInternal(userReference, parameters);
         }
         return result;
     }

xwiki-platform-core/xwiki-platform-user/xwiki-platform-user-default/src/test/java/org/xwiki/user/internal/document/DocumentStringUserReferenceSerializerTest.java

@@ -111,4 +111,12 @@ void serializeCurrentUserReferenceWhenNoCurrentUser()
 
         assertEquals("XWiki.XWikiGuest", this.serializer.serialize(CurrentUserReference.INSTANCE));
     }
+
+    @Test
+    void serializeCurrentUserReferenceWhenCurrentUserIsSuperAdmin()
+    {
+        when(this.currentUserReferenceUserReferenceResolver.resolve(null)).thenReturn(SuperAdminUserReference.INSTANCE);
+
+        assertEquals("XWiki.superadmin", this.serializer.serialize(CurrentUserReference.INSTANCE));
+    }
 }