Fix: GPU temperature and load sensor fix

[amadeo-alex]

The current implementation of GPU temperature and load sensor does not adhere to the HA MQTT standard and can return string "NotSupported" or "Unknown" values which causes HA to produce an error/warning log - value of the state topic should always be a "convertible to number" value.

This PR does three things:

• allows sensors to return "null" value from "GetState()" and "GetAttributes()" functions
• in case "null" is returned for state, no update is sent (if attributes are "null" while state is not, only state is sent)
• changes the "GpuTemperatureSensor" and "GpuLoadSensor" to return null if information cannot be obtained

Should remediate LAB02-Research/HASS.Agent#320 and partially remediate LAB02-Research/HASS.Agent#310 .