fix(detectors): Fix TypeError in N+1 Experimental Detector (#95363)

this pr fixes a type error in the detector

Fixes SENTRY-FOR-SENTRY-5PT6

Author: roggenkemper

src/sentry/performance_issues/detectors/experiments/n_plus_one_db_span_detector.py

@@ -280,7 +280,9 @@ def get_valid_db_span_description(span: Span) -> str | None:
     # Trigger pathway on `mongodb`, `mongoose`, etc...
     if "mongo" in db_system:
         description = span.get("sentry_tags", {}).get("description")
-        return description if "{" in description else None
+        if not description or "{" not in description:
+            return None
+        return description
     return default_description