Fix inconsistencies in getTSConfig in BackenuserAuth
BackenuserAuthentication::getTSConfig() behaves inconsistently when an
object string is given with multiple sections but no result in the actual
TSConfig of the user. Other than noted in the method declaration, the method
returns an empty array instead of an array where the 'properties' key is null.
Additionally we fix a bug which caused wrong results if "0" was used
as a key in configuration like "permissions.file.0"