don't enforce sequential completion of zero requirements

test plan:
 - create a public course
 - create a module in the course
 - add an item
 - add a completion requirement for the item, and check the
   "require sequential progress" option
 - remove the requirement
 - ensure both a logged-in student and a non-logged-in user
   can access the module item

fixes CNVS-24908

Change-Id: Ia692a5a433c9fa4345f65664a650a01e5ada84e0
Reviewed-on: https://gerrit.instructure.com/67204
Tested-by: Jenkins
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Ryan Allen <rallen@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>

app/models/context_module.rb

@@ -237,7 +237,8 @@ class ContextModule < ActiveRecord::Base
   end
 
   def available_for?(user, opts={})
-    return true if self.active? && !self.to_be_unlocked && self.prerequisites.blank? && !self.require_sequential_progress
+    return true if self.active? && !self.to_be_unlocked && self.prerequisites.blank? &&
+      (self.completion_requirements.empty? || !self.require_sequential_progress)
     if self.grants_right?(user, :read_as_admin)
       return true
     elsif !self.active?

spec/models/context_module_spec.rb

@@ -29,6 +29,13 @@ describe ContextModule do
       course_module
       expect(@module.available_for?(nil)).to eql(true)
     end
+
+    it "returns true by default when require_sequential_progress is true and there are no requirements" do
+      course_module
+      @module.require_sequential_progress = true
+      @module.save!
+      expect(@module.available_for?(nil)).to eql(true)
+    end
   end
 
   describe "prerequisites=" do