Fix arg names in language -> pairlist conversion

com.oracle.truffle.r.runtime/src/com/oracle/truffle/r/runtime/data/RPairList.java

@@ -560,7 +560,9 @@ public final class RPairList extends RSharingAttributeStorage implements RAbstra
         Object p = this;
         for (int i = 0; i < newNames.getLength() && !isNull(p); i++) {
             RPairList pList = (RPairList) p;
-            pList.setTag(RDataFactory.createSymbol(newNames.getDataAt(i)));
+            String newNameVal = newNames.getDataAt(i);
+            Object newTag = newNameVal.isEmpty() ? RNull.instance : RDataFactory.createSymbolInterned(newNameVal);
+            pList.setTag(newTag);
             p = pList.cdr();
         }
     }

com.oracle.truffle.r.test.native/packages/testrffi/testrffi/tests/pairlists.R

@@ -30,3 +30,8 @@ l
 api.SET_TYPEOF(l, 6) # LANGSXP
 l
 eval(l)
+
+# language objects treated as pairlists:
+b <- ~fun(arg = val, arg2)
+api.TAG(api.CDR(api.CDR(api.CAR(api.CDR(b)))))
+