Ensure getUserName cannot return null

A user can be created with an explicitly null name, as happens in tests.
But getUserName claims to never return null. So we must do a null-check
here to prevent it.

Bug: 227624966
Test: atest UserManagerTest
Change-Id: Iea0e7b6292c6dd49df1bebc5467091a82ddaedb5
4 files changed