Fix build against Gerrit 'stable-3.7'+ versions

The following error prone issues was fixed:
prone error:
  UserResolver.java:43: error: [ImpossibleNullComparison] This value cannot be null, and comparing it to null may be misleading.
    ? Optional.ofNullable(user.get().getUserName().get())
                         ^

Change-Id: Id82b016299aa7e4a8162e15e57e7b0fd7068f531
1 file changed