Quiz with password start attempt page has blank label

Issue
Form labels - There is an empty label for the text "To attempt this quiz you need to know the quiz password" on the start quiz attempt page when a quiz requires a password.

Standard Level
WCAG 2 1.3.1 (A) http://www.w3.org/TR/UNDERSTANDING-WCAG20/content-structure-separation-programmatic.html

Impact
Serious

Example Link
http://demo.moodle.net/mod/quiz/startattempt.php

Test Steps

1. Create a quiz that requires a password (no need to set up browser security or open in new window)
2. Login as a student
3. Navigate to the course
4. Click on the quiz
5. Click the attempt button
6. Inspect the source code for the text "To attempt this quiz you need to know the quiz password".