Fix potential NPE when generating accessibility text.

Traced through the original bug report; the problem is that a call to
TextUtils.expandTemplate was being made with a null 2nd parameter.
I traced through the callers to find out where a null could be introduced
and I was unable to identify a potential source.  This change introduces
a last-ditch null check to protect against problems upstream.

Bug: 18684529
Change-Id: I7b608dfedf052d6933e94d1f23a658763b66059a