Fix stream events/log with some kinds of non addable output (#17205)

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libs/core/langchain_core/tracers/log_stream.py

@@ -572,6 +572,7 @@ async def _astream_log_implementation(
                     try:
                         final_output = final_output + chunk  # type: ignore
                     except TypeError:
+                        prev_final_output = None
                         final_output = chunk
                 patches: List[Dict[str, Any]] = []
                 if with_streamed_output_list: