Revert incorrect fix from https://review.opencontrail.org/#/c/35856/

There were two issues with the fix made before.

1. All spaces inside link attribute was erased, which is clearly incorrect.
2. JSON supports NULL already. We need to check for null and then set
   the link attribute as an empty object if attr data turns out to be null.

Change-Id: I8cdf876196a709571ee4b763c19af7a0d81e85f8
Closes-Bug: 1773197

config-client-mgr/config_cass2json_adapter.cc

@@ -160,18 +160,18 @@ void ConfigCass2JsonAdapter::AddOneEntry(Value *jsonObject,
             IsLinkWithAttr(obj_type, ref_type);
         if (link_with_attr) {
             Document ref_document(&a);
-            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());
+            ref_document.Parse<0>(c.value.c_str());
             CONFIG_PARSE_ASSERT(ReferenceLinkAttributes,
                                 !ref_document.HasParseError());
             CONFIG_PARSE_ASSERT(ReferenceLinkAttributes,
                                 ref_document.HasMember("attr"));
             Value &attr_value = ref_document["attr"];
-            v.AddMember("attr", attr_value, a);
+            if (!attr_value.IsNull()) {
+                v.AddMember("attr", attr_value, a);
+            } else {
+                Value vm;
+                v.AddMember("attr", vm.SetObject(), a);
+            }
         } else {
             Value vm;
             v.AddMember("attr", vm.SetObject(), a);