Quiz overview screen show reports button with empty label

Issue
Form Labels - Inappropriate use of label on the show report button in the Attempts overview report (considered an orphan label).

Standard Level
WCAG 2 1.1.1 (A) http://www.w3.org/WAI/WCAG20/quickref/#qr-text-equiv-all

Impact
Serious

Example Link
http://demo.moodle.net/mod/quiz/report.php?id=2461&mode=overview

Test Steps

1. Login as student
2. Access the demo course
3. Quiz must be created
4. Attempt must be made
5. Click on quiz link
6. Click the attempts link
7. View the source with the show report button label.