Make GridFrequency produce None when the value is nan

To match the resampler behaviour it is better to convert a `nan` sample
to `None`.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/sdk/timeseries/_grid_frequency.py

@@ -7,7 +7,7 @@
 
 import asyncio
 import logging
-from typing import TYPE_CHECKING, cast
+from typing import TYPE_CHECKING
 
 from frequenz.channels import Receiver, Sender
 
@@ -105,8 +105,8 @@ def new_receiver(self) -> Receiver[Sample[Frequency]]:
             )
 
         return receiver.map(
-            lambda sample: cast(Sample[Frequency], sample)
-            if sample.value is None
+            lambda sample: Sample[Frequency](sample.timestamp, None)
+            if sample.value is None or sample.value.isnan()
             else Sample(sample.timestamp, Frequency.from_hertz(sample.value.base_value))
         )