Add nil checks (#234)

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aws-opentelemetry-distro/src/amazon/opentelemetry/distro/attribute_propagating_span_processor.py

@@ -66,9 +66,9 @@ def on_start(self, span: Span, parent_context: Optional[Context] = None) -> None
         if is_local_root(span):
             if not _is_server_kind(span):
                 propagation_data = self._propagation_data_extractor(span)
-        elif _is_server_kind(parent_span):
+        elif parent_span and _is_server_kind(parent_span):
             propagation_data = self._propagation_data_extractor(parent_span)
-        else:
+        elif parent_span:
             propagation_data = parent_span.attributes.get(self._propagation_data_key)
 
         if propagation_data is not None: