Fix RFFI CAR for empty '...' returns NULL

com.oracle.truffle.r.ffi.impl/src/com/oracle/truffle/r/ffi/impl/nodes/ListAccessNodes.java

@@ -63,6 +63,9 @@ public final class ListAccessNodes {
 
         @Specialization
         protected Object car(RArgsValuesAndNames args) {
+            if (args.isEmpty()) {
+                return RNull.instance;
+            }
             return args.getArgument(0);
         }
 

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

+# Some more tests that work with pairlists and related objects like '...' or language
+stopifnot(require(testrffi))
+
+# Note: GNU R returns the promise, FastR the value, we check that at least there are not exceptions
+foo <- function(...) api.CAR(get('...'))
+invisible(foo(a=3))
+invisible(foo(a=4, b=6))
+
+foo <- function(...) api.CDR(get('...'))
+is.null(foo(a=3))
+names(foo(a=4, b=6))