DEV: improves error message when sso param is not passed (#8549)

SingleSignOnProvider is expecting a sso param later in the chain. If sso param is not found it will cause a 500 with the following exception: `NoMethodError (undefined method `unpack1' for nil:NilClass)` as `set_return_sso_url` is attempting to decode it: https://github.com/discourse/discourse/blob/master/lib/single_sign_on_provider.rb#L19

app/controllers/session_controller.rb

@@ -41,10 +41,12 @@ class SessionController < ApplicationController
   end
 
   def sso_provider(payload = nil)
-    payload ||= request.query_string
-
     if SiteSetting.enable_sso_provider
       begin
+        if !payload
+          params.require(:sso)
+          payload = request.query_string
+        end
         sso = SingleSignOnProvider.parse(payload)
       rescue SingleSignOnProvider::BlankSecret
         render plain: I18n.t("sso.missing_secret"), status: 400