Genex: Restore REMOVE_DUPLICATES preservation of empty elements

Refactoring in commit 72d116ee68 (GenEx: list oriented genexes use
cmList class, 2023-03-29, v3.27.0-rc1~205^2) accidentally caused
empty elements to be dropped by the `REMOVE_DUPLICATES` genex.
Fix it and add a test case.

Fixes: #25080

Source/cmGeneratorExpressionNode.cxx

@@ -384,7 +384,9 @@ static const struct RemoveDuplicatesNode : public cmGeneratorExpressionNode
         "$<REMOVE_DUPLICATES:...> expression requires one parameter");
     }
 
-    return cmList{ parameters.front() }.remove_duplicates().to_string();
+    return cmList{ parameters.front(), cmList::EmptyElements::Yes }
+      .remove_duplicates()
+      .to_string();
   }
 
 } removeDuplicatesNode;