fix submitting comments on attempt 0 in a2

when submitted comments for attempt 0 (the case where we have
no submissions) the attempt was reading as nil in the validation
and failing to create the comment.

Test Plan:
* as a teacher, create an assignment
* as a student in this assignment who has not yet submitted,
  navigate to the Comments tab
* create a new comment on the assignment
* it should successfully create the comment
* comments for attempt 0 and attempt 1 should display
  together

* on an assignment with one or more submissions, create a
  comment
* it should successfully create the comment

fixes COMMS-2174

Change-Id: If216a541aebf7db2ea7b6942a02d194113cd8e41
Reviewed-on: https://gerrit.instructure.com/199377
Reviewed-by: Steven Burnett <sburnett@instructure.com>
QA-Review: Michelle Simmons <misimmons@instructure.com>
Tested-by: Jenkins
Product-Review: Ryan Norton <rnorton@instructure.com>

app/models/submission_comment.rb

@@ -49,7 +49,7 @@ class SubmissionComment < ActiveRecord::Base
   validates_length_of :comment, :minimum => 1, :allow_nil => true, :allow_blank => true
   validates_each :attempt do |record, attr, value|
     next if value.nil?
-    if record.submission.attempt.nil? || value > record.submission.attempt
+    if value > (record.submission.attempt || 0)
       record.errors.add(attr, 'attempt must not be larger than number of submission attempts')
     end
   end

spec/models/submission_comment_spec.rb

@@ -648,6 +648,20 @@ This text has a http://www.google.com link in it...
       @comment4 = @submission.submission_comments.create!(valid_attributes.merge(attempt: nil))
     end
 
+    context 'when the submission attempt is nil' do
+      before(:once) do
+        @submission.update!(attempt: nil)
+      end
+
+      it 'raises an error if the submission_comment attempt is greater than 0' do
+        expect { @submission.submission_comments.create!(valid_attributes.merge(attempt: 1)) }.to raise_error(ActiveRecord::RecordInvalid)
+      end
+
+      it 'does not raise an error if the submission_comment attempt is equal to 0' do
+        expect { @submission.submission_comments.create!(valid_attributes.merge(attempt: 0)) }.not_to raise_error
+      end
+    end
+
     it 'can limit comments to the specific attempt' do
       expect(@submission.submission_comments.where(attempt: 1)).to eq [@comment1]
     end
@@ -660,14 +674,14 @@ This text has a http://www.google.com link in it...
       expect(@submission.submission_comments.where(attempt: nil)).to eq [@comment4]
     end
 
-    it 'cannot be present? if submisssion#attempt is nil' do
+    it 'cannot be present? if submission#attempt is nil' do
       @submission.update_column(:attempt, nil) # bypass infer_values callback
       @comment1.reload
       @comment1.attempt = 2
       expect(@comment1).not_to be_valid
     end
 
-    it 'cannot be larger then submisssion#attempt' do
+    it 'cannot be larger then submission#attempt' do
       @comment1.attempt = @submission.attempt + 1
       expect(@comment1).not_to be_valid
     end