Fix NPE in UserManager.getUserName(). Unlike all other calls to

getUserInfo(), getUserName() was the only one that did not check for
null. This has led to NPEs like in b/37589362.
Test: none. :( Not sure how to force the race where getUserInfo()
returns null. Verified manually that getUserName() still works.

Change-Id: I98ef06fe99ba760ae0194ec256fc9d1f39d3b7e5