Fix _otel lint

Technically, `span.otel_span` can be None, which is not accepted by
`trace.use_span`.

elasticsearch_serverless/_otel.py

@@ -106,7 +106,7 @@ def helpers_span(self, span_name: str) -> Generator[OpenTelemetrySpan, None, Non
 
     @contextlib.contextmanager
     def use_span(self, span: OpenTelemetrySpan) -> Generator[None, None, None]:
-        if not self.enabled or self.tracer is None:
+        if not self.enabled or self.tracer is None or span.otel_span is None:
             yield
             return