Fix _send_temperatures() for Seneca device

As the Seneca device now returns its temperatures as a numpy array,
unfortunately the _send_temperatures() function no longer works with it:

```
Traceback (most recent call last):
  File "/home/alex/code/FINESSE/finesse/gui/temp_control.py", line 265, in _poll_dp9800
    pub.sendMessage(f"device.{TEMPERATURE_MONITOR_TOPIC}.data.request")
  File "/home/alex/code/FINESSE/.venv/lib/python3.11/site-packages/pubsub/core/publisher.py", line 216, in sendMessage
    topicObj.publish(**msgData)
  File "/home/alex/code/FINESSE/.venv/lib/python3.11/site-packages/pubsub/core/topicobj.py", line 452, in publish
    self.__sendMessage(msgData, topicObj, msgDataSubset)
  File "/home/alex/code/FINESSE/.venv/lib/python3.11/site-packages/pubsub/core/topicobj.py", line 482, in __sendMessage
    listener(data, self, allData)
  File "/home/alex/code/FINESSE/.venv/lib/python3.11/site-packages/pubsub/core/listener.py", line 237, in __call__
    cb(**kwargs)
  File "/home/alex/code/FINESSE/finesse/hardware/__init__.py", line 47, in _send_temperatures
    temperatures = _try_get_temperatures() or _DEFAULT_TEMPS
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()
```

Seemingly unlike Python's built-in collections, you can't check whether
a numpy array is empty by looking at its "truthiness".

Fix by using an explicit if-statement.

finesse/hardware/__init__.py

@@ -44,7 +44,10 @@ def _try_get_temperatures() -> Sequence | None:
 
 def _send_temperatures() -> None:
     """Send the current temperatures (or NaNs) via pubsub."""
-    temperatures = _try_get_temperatures() or _DEFAULT_TEMPS
+    temperatures = _try_get_temperatures()
+    if temperatures is None:
+        temperatures = _DEFAULT_TEMPS
+
     time = datetime.utcnow()
     pub.sendMessage(
         f"device.{TEMPERATURE_MONITOR_TOPIC}.data.response",