fix(detectors): Fix AttributeError in Render Blocking Asset detector (#95366)

Fixes SENTRY-FOR-SENTRY-5SYS

Author: roggenkemper

src/sentry/performance_issues/detectors/render_blocking_asset_span_detector.py

@@ -124,7 +124,7 @@ def _get_duration(self, item: Mapping[str, Any] | None) -> float:
     def _is_blocking_render(self, span: Span) -> bool:
         assert self.fcp is not None
 
-        data = span.get("data", {})
+        data = span.get("data", {}) or {}
         render_blocking_status = data.get("resource.render_blocking_status")
         if render_blocking_status == "non-blocking":
             return False