Return an empty string rather than None when calling repr()

Repr() should return "a string containing a printable representation of
an object," which is a convention Sphinx 1.2 relies on when building
the docs.

Fixes bug 1163369

Change-Id: Ieefd69c5b4b521500679df7745f60cb3a4d2f77c
(cherry picked from commit 29cb3251afd65fdd0a98b76d58f05458aaab3b47)