fix 500 on site credits page (bug 1011849)

fixes [bug 1011849](https://bugzilla.mozilla.org/show_bug.cgi?id=1011849)

This was introduced by a change in https://github.com/mozilla/olympia/commit/791b698efb38d450612e0953786b4713adad9f2c#diff-cbd0714d6d3c27ca05dd66526fcb4ad7R44