Fix STT model name attribute retrieval in tracing decorator

Changed getattr with default value to use 'or' operator for fallback.
This ensures proper model name retrieval when model_name attribute exists but is None or empty.

src/pipecat/utils/tracing/service_decorators.py

@@ -303,7 +303,7 @@ def traced_stt(func: Optional[Callable] = None, *, name: Optional[str] = None) -
                         add_stt_span_attributes(
                             span=current_span,
                             service_name=service_class_name,
-                            model=getattr(self, "model_name", settings.get("model", "unknown")),
+                            model=getattr(self, "model_name") or settings.get("model", "unknown"),
                             transcript=transcript,
                             is_final=is_final,
                             language=str(language) if language else None,