Fix bug in GridFrequency quantity conversion

The sample value can also be `None`, in which case we can't access to
`sample.value.base_value`. In that case we just cast the existing sample
as it doesn't matter which type the value has when it is `None`,
avoiding the extra copying.

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
+from typing import TYPE_CHECKING, cast
 
 from frequenz.channels import Receiver, Sender
 
@@ -105,9 +105,9 @@ def new_receiver(self) -> Receiver[Sample[Frequency]]:
             )
 
         return receiver.map(
-            lambda sample: Sample(
-                sample.timestamp, Frequency.from_hertz(sample.value.base_value)
-            )
+            lambda sample: cast(Sample[Frequency], sample)
+            if sample.value is None
+            else Sample(sample.timestamp, Frequency.from_hertz(sample.value.base_value))
         )
 
     async def _send_request(self) -> None: