logs: handle None in _encode_value

[xrmx]

Is your feature request related to a problem?

#4276 adding a workaround for serializing logs with empy body.

Describe the solution you'd like

Since the addition of null as valid value for Any may have been added after our implementation we may want to update the opentelemetry.exporter.otlp.proto.common._internal._encode_value to handle None gracefully.

Describe alternatives you've considered

No response

Additional Context

No response

Would you like to implement a fix?

None