OpusInterface: Set initial status to None

Previously we were using `SpectrometerStatus.UNDEFINED` as the initial value, but this is a bad idea because:

1. It's a bit ugly
2. The server could conceivably return `UNDEFINED` as a status, which would break our code -- probably best not to rely on this not happening

finesse/hardware/plugins/spectrometer/opus_interface.py

@@ -96,7 +96,7 @@ def __init__(
         self._url = f"http://{host}:{port}/opusrs"
         """URL to make requests."""
 
-        self._status = SpectrometerStatus.UNDEFINED
+        self._status: SpectrometerStatus | None = None
         """The last known status of the spectrometer."""
         self._status_timer = QTimer()
         self._status_timer.timeout.connect(self._request_status)
@@ -124,7 +124,7 @@ def _on_reply_received(self, reply: QNetworkReply) -> None:
         # If the status has changed, notify listeners
         if new_status != self._status:
             # On first update, we need to signal that the device is now open
-            if self._status == SpectrometerStatus.UNDEFINED:
+            if self._status is None:
                 self.signal_is_opened()
 
             self._status = new_status

tests/hardware/plugins/spectrometer/test_opus_interface.py

@@ -36,7 +36,7 @@ def test_init(timer_mock: Mock, subscribe_mock: Mock) -> None:
         assert opus._url == f"http://{DEFAULT_OPUS_HOST}:{DEFAULT_OPUS_PORT}/opusrs"
         status_mock.assert_called_once_with()
 
-        assert opus._status == SpectrometerStatus.UNDEFINED
+        assert opus._status is None
         timer.setSingleShot.assert_called_once_with(True)
         timer.setInterval.assert_called_once_with(1000)
         timer.timeout.connect.assert_called_once_with(opus._request_status)