Add null checks when registering backup transport

Problem: {

//frameworks/base/services/backup/backuplib/java/com/android/server/backup/TransportManager.java
NullPointerException is occurring as a transport with null transportDirName is attempted to register,
causing subsequent fatal in system server resulting in restart.

Bug : https://partnerissuetracker.corp.google.com/issues/298000577

}

Solution: {
  Null check transportName and transportDirName added to circumvent fatal in system
}

Bug: 298000577
Change-Id: I865dbc14768fb9be3e557261aab05b1dc3dc5839