Return None when value is not known in OpenHardwareMonitor (#101127)

* Return None when value is not known

* Add to coverage

.coveragerc

@@ -898,6 +898,7 @@ omit =
     homeassistant/components/opengarage/cover.py
     homeassistant/components/opengarage/entity.py
     homeassistant/components/opengarage/sensor.py
+    homeassistant/components/openhardwaremonitor/sensor.py
     homeassistant/components/openhome/__init__.py
     homeassistant/components/openhome/const.py
     homeassistant/components/openhome/media_player.py

homeassistant/components/openhardwaremonitor/sensor.py

@@ -79,6 +79,8 @@ class OpenHardwareMonitorDevice(SensorEntity):
     @property
     def native_value(self):
         """Return the state of the device."""
+        if self.value == "-":
+            return None
         return self.value
 
     @property