Fixes CN crash when ref attribute is null

Added check for attr "null" and handle it gracefully by forcing it to
be an empty set {}
Modified existing test data to test this case.
Closes-bug: 1713407

Change-Id: I035729e2dc0e8d403c55ffc032d6c49001a68e5f

src/config/config-client-mgr/config_cass2json_adapter.cc

@@ -159,7 +159,12 @@ void ConfigCass2JsonAdapter::AddOneEntry(Value *jsonObject,
             IsLinkWithAttr(obj_type, ref_type);
         if (link_with_attr) {
             Document ref_document(&a);
-            ref_document.Parse<0>(c.value.c_str());
+            string c_value = c.value;
+            c_value.erase(remove(c_value.begin(), c_value.end(), ' '),
+                          c_value.end());
+            if (c_value == "{\"attr\":null}")
+                c_value = "{\"attr\":{}}";
+            ref_document.Parse<0>(c_value.c_str());
             CONFIG_PARSE_ASSERT(ReferenceLinkAttributes,
                                 !ref_document.HasParseError());
             CONFIG_PARSE_ASSERT(ReferenceLinkAttributes,

src/ifmap/testdata/cli1_vn2_np1_add.json

@@ -295,7 +295,7 @@
       "d0116475-a0a8-472d-9743-314d0e94c9e9": {
         "fq_name": "[\"default-domain\",\"demo\",\"vn27-1\"]",
         "type": "\"virtual_network\"",
-        "ref:network_policy:7367cbbc-8036-445d-9f7c-4190c91cdf5d": "{\"attr\":{\"major\":\"0\",\"minor\":\"0\"}}",
+        "ref:network_policy:7367cbbc-8036-445d-9f7c-4190c91cdf5d": "{\"attr\": null}",
         "ref:network_ipam:9963c319-fadd-4b8d-b3f8-7160e682e2f6": "{\"attr\":{\"ipam_subnets\":[{\"subnet\":{\"ip_prefix\":\"27.1.1.0\",\"ip_prefix_len\":\"24\"},\"default_gateway\":\"27.1.1.254\"}]}}",
         "prop:id_perms": "{\"permissions\":{\"owner\":\"cloud-admin\",\"owner_access\":\"7\",\"group\":\"cloud-admin-group\",\"group_access\":\"7\",\"other_access\":\"7\"},\"uuid\":{\"uuid_mslong\":\"14992875090862425901\",\"uuid_lslong\":\"10899609730193607145\"},\"enable\":\"true\"}",
         "parent:project:ee523edc-e957-41ab-b7b0-2d557e9573ff": "null"