add IPv6 routing tests converted from SAI PTF

common/sai.py

@@ -286,6 +286,8 @@ def create(self, obj, attrs=[], do_assert=True):
         return self.sai_client.create(obj, attrs, do_assert)
 
     def remove(self, obj, do_assert=True):
+        if obj is None:
+            return
         return self.sai_client.remove(obj, do_assert)
 
     def set(self, obj, attr, do_assert=True):

common/sai_npu.py

@@ -168,6 +168,8 @@ def create_route(self, dest, vrf_oid, nh_oid=None, opt_attr=None):
                    ), attrs)
 
     def remove_route(self, dest, vrf_oid):
+        if dest is None or vrf_oid is None:
+            return
         self.remove('SAI_OBJECT_TYPE_ROUTE_ENTRY:' + json.dumps(
                        {
                            "dest":      dest,