MSFT Sync: Don't send empty string as description

Microsoft API doesn't like it

Test plan:
- pretty trivial, specs are probably enough.

Change-Id: I1720cd9f0b78b7189a63c205c21bb5299d0840e6
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/262050
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Reviewed-by: Weston Dransfield <wdransfield@instructure.com>
QA-Review: Weston Dransfield <wdransfield@instructure.com>
Product-Review: Evan Battaglia <ebattaglia@instructure.com>

lib/microsoft_sync/canvas_graph_service.rb

@@ -39,7 +39,7 @@ module MicrosoftSync
     # Returns the hash of the new course, including the 'id' key
     def create_education_class(course)
       graph_service.create_education_class(
-        description: course.public_description,
+        description: course.public_description.presence,
         displayName: course.name,
         externalId: course.uuid,
         externalName: course.name,

spec/lib/microsoft_sync/canvas_graph_service_spec.rb

@@ -53,10 +53,21 @@ describe MicrosoftSync::CanvasGraphService do
         mailNickname: "Course_math_101-#{@course.uuid.first(13)}",
       ).and_return('foo')
 
-
       expect(subject.create_education_class(@course)).to eq('foo')
     end
 
+    context 'when the course has a empty string for a description' do
+      # Microsoft API seems to have a problem with description being an empty
+      # string
+      it 'sends nil instead of the empty string' do
+        course_model(public_description: '', name: 'math 101')
+        expect(graph_service).to receive(:create_education_class).with(
+          hash_including(description: nil)
+        ).and_return('foo')
+        subject.create_education_class(@course)
+      end
+    end
+
     context 'when the course code contains special characters' do
       let(:course_code) { '{{math🔥一241!&?' }