fix bug where url draft attempt crashes

When a user saves a draft URL on the third attempt and then attempts to
navigate to the first attempt the page would crash.

Test Plan:
- Create a course and add a student to it
- Create an assignment with an online url submission type
- As the student navigate to the assignment and create and submit 2
  drafts
- Create a third draft and then attempt to navigate to the first draft
- Note that the page no longer crashes

fixes KNO-145
flag=assignments_2_student

Change-Id: I1c75d4e08d43f0f282c4fec5329b08f97d09ce4e
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/217913
Tested-by: Service Cloud Jenkins <svc.cloudjenkins@instructure.com>
Tested-by: Jenkins
Product-Review: Matthew Lemon <mlemon@instructure.com>
Reviewed-by: Steven Burnett <sburnett@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
QA-Review: Steven Burnett <sburnett@instructure.com>

app/jsx/assignments_2/student/components/AttemptType/UrlEntry.js

@@ -46,7 +46,8 @@ class UrlEntry extends React.Component {
 
   componentDidUpdate(prevProps) {
     if (
-      this.props.submission?.submissionDraft?.url !== prevProps.submission?.submissionDraft?.url
+      this.props.submission?.submissionDraft?.url &&
+      this.props.submission.submissionDraft.url !== prevProps.submission?.submissionDraft?.url
     ) {
       this.updateInputState()
     }