feat: Link help center portal to an Inbox (#6903)

app/controllers/api/v1/accounts/inboxes_controller.rb

@@ -114,10 +114,13 @@ class Api::V1::Accounts::InboxesController < Api::V1::Accounts::BaseController
   def inbox_attributes
     [:name, :avatar, :greeting_enabled, :greeting_message, :enable_email_collect, :csat_survey_enabled,
      :enable_auto_assignment, :working_hours_enabled, :out_of_office_message, :timezone, :allow_messages_after_resolved,
-     :lock_to_single_conversation]
+     :lock_to_single_conversation, :portal_id]
   end
 
   def permitted_params(channel_attributes = [])
+    # We will remove this line after fixing https://linear.app/chatwoot/issue/CW-1567/null-value-passed-as-null-string-to-backend
+    params.each { |k, v| params[k] = params[k] == 'null' ? nil : v }
+
     params.permit(
       *inbox_attributes,
       channel: [:type, *channel_attributes]