SingleUserGroup: Use AccountCache#getOrNull instead of AccountCache#get

If an account doesn't exist AccountCache#get returns an empty
AccountState instance to represent the missing account. We don't need
single user groups for missing accounts, hence use
AccountCache#getOrNull instead.

Change-Id: Ie6b1073979bc26c8f960fb01cf63ade79d60f4d4
Signed-off-by: Edwin Kempin <ekempin@google.com>