unique(NULL) specialization

com.oracle.truffle.r.nodes.builtin/src/com/oracle/truffle/r/nodes/builtin/base/Unique.java

@@ -48,6 +48,12 @@ public abstract class Unique extends RBuiltinNode {
                         ConstantNode.create(RMissing.instance)};
     }
 
+    @SuppressWarnings("unused")
+    @Specialization
+    public RNull doUnique(RNull vec, byte incomparables, byte fromLast, byte nmax, RMissing vararg) {
+        return vec;
+    }
+
     @SuppressWarnings("unused")
     @Specialization
     public RStringVector doUnique(RAbstractStringVector vec, byte incomparables, byte fromLast, byte nmax, RMissing vararg) {