Reviewing a version fails if summary field is suppressed.

Steps to reproduce:

1. Edit a page as IP
2. Login as administrator and suppress the summary of this revision
3. Try to review the page.

Error message: "Unable to review this revision. The target revision does not exist."

Testcase: https://de.wikipedia.beta.wmflabs.org/w/index.php?title=Test&diff=16010&oldid=16009

This bug was reported from the German Wikipedia: https://de.wikipedia.org/w/index.php?title=Monica_Seles&action=history
The edit on 2016-09-13 itself was good but the summary only was bad and therefore suppresssed.