handle missing assignment date values for variable subs

fixes PLAT-1272

test plan:

use the $Canvas.assignment.(dueAt|lockAt|unlockAt).iso8601 
varaible expansions

set up an assignment with blank values for all the dates

you should not get a 500 on an lti launch for the assignment

Change-Id: I7e4ffab74ea8b2aef6ba921c372ada9260805da0
Reviewed-on: https://gerrit.instructure.com/65354
Tested-by: Jenkins
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Nathan Mills <nathanm@instructure.com>

lib/lti/variable_expander.rb

@@ -306,15 +306,15 @@ module Lti
 
     register_expansion 'Canvas.assignment.unlockAt.iso8601', [],
                        -> { @assignment.unlock_at.utc.iso8601 },
-                       ASSIGNMENT_GUARD
+                       -> {@assignment && @assignment.unlock_at.present?}
 
     register_expansion 'Canvas.assignment.lockAt.iso8601', [],
                        -> { @assignment.lock_at.utc.iso8601 },
-                       ASSIGNMENT_GUARD
+                       -> {@assignment && @assignment.lock_at.present?}
 
     register_expansion 'Canvas.assignment.dueAt.iso8601', [],
                        -> { @assignment.due_at.utc.iso8601 },
-                       ASSIGNMENT_GUARD
+                       -> {@assignment && @assignment.due_at.present?}
 
     register_expansion 'LtiLink.custom.url', [],
                        -> { @controller.show_lti_tool_settings_url(@tool_setting_link_id) },

spec/lib/lti/variable_expander_spec.rb

@@ -380,27 +380,51 @@ module Lti
           subject.expand_variables!(exp_hash)
           expect(exp_hash[:test]).to eq right_now.to_s
         end
+        context 'iso8601' do
+          it 'has substitution for $Canvas.assignment.unlockAt.iso8601' do
+            assignment.stubs(:unlock_at).returns(right_now)
+            exp_hash = {test: '$Canvas.assignment.unlockAt.iso8601'}
+            subject.expand_variables!(exp_hash)
+            expect(exp_hash[:test]).to eq right_now.utc.iso8601.to_s
+          end
+
+          it 'has substitution for $Canvas.assignment.lockAt.iso8601' do
+            assignment.stubs(:lock_at).returns(right_now)
+            exp_hash = {test: '$Canvas.assignment.lockAt.iso8601'}
+            subject.expand_variables!(exp_hash)
+            expect(exp_hash[:test]).to eq right_now.utc.iso8601.to_s
+          end
+
+          it 'has substitution for $Canvas.assignment.dueAt.iso8601' do
+            assignment.stubs(:due_at).returns(right_now)
+            exp_hash = {test: '$Canvas.assignment.dueAt.iso8601'}
+            subject.expand_variables!(exp_hash)
+            expect(exp_hash[:test]).to eq right_now.utc.iso8601.to_s
+          end
+
+          it 'handles a nil unlock_at' do
+            assignment.stubs(:unlock_at).returns(nil)
+            exp_hash = {test: '$Canvas.assignment.unlockAt.iso8601'}
+            subject.expand_variables!(exp_hash)
+            expect(exp_hash[:test]).to eq "$Canvas.assignment.unlockAt.iso8601"
+          end
+
+          it 'handles a nil lock_at' do
+            assignment.stubs(:lock_at).returns(nil)
+            exp_hash = {test: '$Canvas.assignment.lockAt.iso8601'}
+            subject.expand_variables!(exp_hash)
+            expect(exp_hash[:test]).to eq "$Canvas.assignment.lockAt.iso8601"
+          end
+
+          it 'handles a nil due_at' do
+            assignment.stubs(:lock_at).returns(nil)
+            exp_hash = {test: '$Canvas.assignment.dueAt.iso8601'}
+            subject.expand_variables!(exp_hash)
+            expect(exp_hash[:test]).to eq "$Canvas.assignment.dueAt.iso8601"
+          end
 
-        it 'has substitution for $Canvas.assignment.unlockAt.iso8601' do
-          assignment.stubs(:unlock_at).returns(right_now)
-          exp_hash = {test: '$Canvas.assignment.unlockAt.iso8601'}
-          subject.expand_variables!(exp_hash)
-          expect(exp_hash[:test]).to eq right_now.utc.iso8601.to_s
         end
 
-        it 'has substitution for $Canvas.assignment.lockAt.iso8601' do
-          assignment.stubs(:lock_at).returns(right_now)
-          exp_hash = {test: '$Canvas.assignment.lockAt.iso8601'}
-          subject.expand_variables!(exp_hash)
-          expect(exp_hash[:test]).to eq right_now.utc.iso8601.to_s
-        end
-
-        it 'has substitution for $Canvas.assignment.dueAt.iso8601' do
-          assignment.stubs(:due_at).returns(right_now)
-          exp_hash = {test: '$Canvas.assignment.dueAt.iso8601'}
-          subject.expand_variables!(exp_hash)
-          expect(exp_hash[:test]).to eq right_now.utc.iso8601.to_s
-        end
 
       end