fix adding new assignment from modules' add item dialog

When no assignment groups exist yet.

Change-Id: I73713f4ccfae9b944b00d0aa1e79af0eeb5d6003
Reviewed-on: https://gerrit.instructure.com/4335
Tested-by: Hudson <hudson@instructure.com>
Reviewed-by: Zach Wily <zach@instructure.com>

app/controllers/assignments_controller.rb

@@ -345,7 +345,7 @@ class AssignmentsController < ApplicationController
 
   def get_assignment_group(assignment_params)
     return unless assignment_params
-    if group_id = assignment_params.delete(:assignment_group_id)
+    if (group_id = assignment_params.delete(:assignment_group_id)).present?
       group = @context.assignment_groups.find(group_id)
     end
   end

spec/controllers/assignments_controller_spec.rb

@@ -173,6 +173,14 @@ describe AssignmentsController do
       assigns[:assignment].title.should eql("some assignment")
       assigns[:assignment].context_id.should eql(@course.id)
     end
+
+    it "should create assignment when no groups exist yet" do
+      course_with_student_logged_in(:active_all => true)
+      post 'create', :course_id => @course.id, :assignment => {:title => "some assignment", :assignment_group_id => ''}
+      assigns[:assignment].should_not be_nil
+      assigns[:assignment].title.should eql("some assignment")
+      assigns[:assignment].context_id.should eql(@course.id)
+    end
   end
   
   describe "GET 'edit'" do