Update type signature of get_metrics_data() to accept "None" return t…

…ype (open-telemetry#4186)

opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/export/__init__.py

@@ -404,7 +404,7 @@ def __init__(
 
     def get_metrics_data(
         self,
-    ) -> "opentelemetry.sdk.metrics.export.MetricsData":
+    ) -> Optional["opentelemetry.sdk.metrics.export.MetricsData"]:
         """Reads and returns current metrics from the SDK"""
         with self._lock:
             self.collect()