if no keys then ListMR should instead of RNull return an empty vector (responding to HAS_SIZE)

com.oracle.truffle.r.engine/src/com/oracle/truffle/r/engine/interop/ListMR.java

@@ -45,12 +45,12 @@ import com.oracle.truffle.r.nodes.access.vector.ReplaceVectorNode;
 import com.oracle.truffle.r.nodes.attributes.SpecialAttributesFunctions.GetNamesAttributeNode;
 import com.oracle.truffle.r.nodes.control.RLengthNode;
 import com.oracle.truffle.r.runtime.data.NativeDataAccess;
+import com.oracle.truffle.r.runtime.data.RDataFactory;
 import com.oracle.truffle.r.runtime.data.RExpression;
 import com.oracle.truffle.r.runtime.data.RFunction;
 import com.oracle.truffle.r.runtime.data.RList;
 import com.oracle.truffle.r.runtime.data.RLogical;
 import com.oracle.truffle.r.runtime.data.RMissing;
-import com.oracle.truffle.r.runtime.data.RNull;
 import com.oracle.truffle.r.runtime.data.RPairList;
 import com.oracle.truffle.r.runtime.data.RStringVector;
 import com.oracle.truffle.r.runtime.data.RTruffleObject;
@@ -533,6 +533,6 @@ public class ListMR {
 
     private static Object listKeys(TruffleObject receiver, GetNamesAttributeNode getNamesNode) {
         RStringVector names = getNamesNode.getNames(receiver);
-        return names != null ? names : RNull.instance;
+        return names != null ? names : RDataFactory.createEmptyStringVector();
     }
 }