Do not try to authenticate with no_input_strategies are empty.

lib/devise/controllers/internal_helpers.rb

@@ -91,8 +91,9 @@ MESSAGE
       # Example:
       #   before_filter :require_no_authentication, :only => :new
       def require_no_authentication
-        args = devise_mapping.no_input_strategies.dup.push :scope => resource_name
-        if warden.authenticate?(*args)
+        no_input = devise_mapping.no_input_strategies
+        args = no_input.dup.push :scope => resource_name
+        if no_input.present? && warden.authenticate?(*args)
           resource = warden.user(resource_name)
           flash[:alert] = I18n.t("devise.failure.already_authenticated")
           redirect_to after_sign_in_path_for(resource)

test/controllers/internal_helpers_test.rb

@@ -45,6 +45,13 @@ class HelpersTest < ActionController::TestCase
     @controller.send :require_no_authentication
   end
 
+  test 'require no authentication skips if no inputs are available' do
+    Devise.mappings[:user].expects(:no_input_strategies).returns([])
+    @mock_warden.expects(:authenticate?).never
+    @controller.expects(:redirect_to).never
+    @controller.send :require_no_authentication
+  end
+
   test 'require no authentication sets a flash message' do
     @mock_warden.expects(:authenticate?).with(:rememberable, :token_authenticatable, :scope => :user).returns(true)
     @mock_warden.expects(:user).with(:user).returns(User.new)