don't try and set a blank sis_user_id for pseudonyms created via the browser

Change-Id: If0a88867faef6b838517d65b70b828fc80715582
Reviewed-on: https://gerrit.instructure.com/4295
Tested-by: Hudson <hudson@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>

app/controllers/pseudonyms_controller.rb

@@ -262,6 +262,7 @@ class PseudonymsController < ApplicationController
       params[:pseudonym][:account] = Account.root_accounts.find(account_id)
     end
     params[:pseudonym][:account] ||= @domain_root_account
+    params[:pseudonym].delete(:sis_user_id) if params[:pseudonym][:sis_user_id].blank?
     @pseudonym = @user.pseudonyms.build(params[:pseudonym])
     unless @pseudonym.account && @pseudonym.account.settings[:admins_can_change_passwords]
       params[:pseudonym].delete :password