Use device duck typing for empty device lists

We are soon going to require all IFRT users to use `Client::MakeDeviceList()` to create a device list instead of calling `BasicDeviceList::Create()`. `PyDeviceList(nb::tuple py_device_assignment)` with an empty device assignment cannot infer the IFRT client to use, so it seems the simplest to fall back to device duck typing like we already do for device lists with devices from multiple clients.

PiperOrigin-RevId: 726233342

xla/python/py_device_list.cc

@@ -56,7 +56,6 @@ PyDeviceList::PyDeviceList(nb::tuple py_device_assignment)
     : device_list_(py_device_assignment) {
   // Attempt to convert to Python devices into `ifrt::DeviceList`.
   if (py_device_assignment.size() == 0) {
-    device_list_ = xla::ifrt::BasicDeviceList::Create({});
     return;
   }
   absl::InlinedVector<xla::ifrt::Device*, 1> devices;