fix: check if days is None before conversion

freqtrade/data/history/history_utils.py

@@ -604,7 +604,7 @@ def download_data(
     Download data function. Used from both cli and API.
     """
     timerange = TimeRange()
-    if "days" in config:
+    if "days" in config and config["days"] is not None:
         time_since = (datetime.now() - timedelta(days=config["days"])).strftime("%Y%m%d")
         timerange = TimeRange.parse_timerange(f"{time_since}-")