fix: be more selective before activating parallel download

Author: xmatthias

freqtrade/data/history/history_utils.py

@@ -474,18 +474,20 @@ def _download_all_pairs_history_parallel(
     :return: Candle pairs with timeframes
     """
     candles: dict[PairWithTimeframe, DataFrame] = {}
-    since = 0
+    since: int | None = None
     if timerange:
         if timerange.starttype == "date":
             since = timerange.startts * 1000
 
     candle_limit = exchange.ohlcv_candle_limit(timeframe, candle_type)
     one_call_min_time_dt = dt_ts(date_minus_candles(timeframe, candle_limit))
     # check if we can get all candles in one go, if so then we can download them in parallel
-    if since > one_call_min_time_dt:
+    if since is None or since > one_call_min_time_dt:
         logger.info(
-            f"Downloading parallel candles for {timeframe} for all pairs "
-            f"since {format_ms_time(since)}"
+            f"Downloading parallel candles for {timeframe} for all pairs"
+            f" since {format_ms_time(since)}"
+            if since
+            else "."
         )
         needed_pairs: ListPairsWithTimeframes = [
             (p, timeframe, candle_type) for p in [p for p in pairs]