Allow None values in OTLP exporter log attributes 

See this comment for context https://github.com/open-telemetry/opentelemetry-python/pull/4400/files/18d51bb83137bceb90bd764412adf61649746f7f#r1932632329

We decided to go ahead and get #4400 merged for release and follow up on this bug separately. We can probably remove `allow_null` altogether.