can use blank collection with grouped_collection input, closes #430

lib/simple_form/inputs/grouped_collection_select_input.rb

@@ -19,7 +19,7 @@ module SimpleForm
 
       # Sample collection
       def collection
-        @collection ||= grouped_collection.first.try(:send, group_method)
+        @collection ||= grouped_collection.first.try(:send, group_method) || []
       end
 
       def group_method

test/inputs/grouped_collection_select_input_test.rb

@@ -20,6 +20,15 @@ class GroupedCollectionSelectInputTest < ActionView::TestCase
     end
   end
 
+  test 'grouped collection accepts empty array collection form' do
+    with_input_for @user, :tag_ids, :grouped_select,
+      :collection => [],
+      :group_method => :last
+
+    assert_select 'select.grouped_select#user_tag_ids'
+  end
+
+
   test 'grouped collection accepts proc as collection' do
     with_input_for @user, :tag_ids, :grouped_select,
       :collection => Proc.new { [['Authors', ['Jose', 'Carlos']], ['General', ['Bob', 'John']]] },