ExtensionMethods.ToCredentialsFromUri would always return null

The method ExtensionMethods.ToCredentialsFromUri would always return
null because it was telling string.Split to only return 1 value.

Once that bug was fixed, it would cause units tests to fail because the
IChangeTrackerClient (specifically Puller) would not always have an
HttpClient.  Added an additional null check to ChangeTracker.