chore: Enable updating conversation additional attributes (#1611)

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

Fixes: #1357

app/models/conversation.rb

@@ -36,6 +36,7 @@ class Conversation < ApplicationRecord
 
   validates :account_id, presence: true
   validates :inbox_id, presence: true
+  before_validation :validate_additional_attributes
 
   enum status: { open: 0, resolved: 1, bot: 2 }
 
@@ -136,6 +137,10 @@ class Conversation < ApplicationRecord
 
   private
 
+  def validate_additional_attributes
+    self.additional_attributes = {} unless additional_attributes.is_a?(Hash)
+  end
+
   def set_bot_conversation
     self.status = :bot if inbox.agent_bot_inbox&.active?
   end