chore: Enable updating conversation additional attributes (#1611)

- Enable updating additional attributes via conversation API
- Handle the case when additional_attributes is nil

Fixes: #1357

swagger/paths/conversation/index_or_create.yml

@@ -100,6 +100,9 @@ post:
           source_id:
             type: string
             description: Contact Source Id
+          additional_attributes:
+            type: object
+            description: Lets you specify attributes like browser information
 
   responses:
     200: