optimization for NULL in IsNamespaceEnv

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

@@ -88,6 +88,11 @@ public class NamespaceFunctions {
         protected byte doIsNamespaceEnv(REnvironment env) {
             return RRuntime.asLogical(env.isNamespaceEnv());
         }
+
+        @Specialization
+        protected byte doIsNamespaceEnv(RNull env) {
+            return RRuntime.LOGICAL_FALSE;
+        }
     }
 
     @RBuiltin(name = "getNamespaceRegistry", kind = INTERNAL, parameterNames = {})