fix: Guard timeframe validation loop against empty list

Return early from `_build_ohlcv_dl_jobs` if `pair_list` is empty.
This prevents unconditional access to `self.timeframes` (which was introduced by the optimization), ensuring that tests or scenarios with uninitialized/mocked exchanges and empty pair lists do not crash.

Co-authored-by: Corax-CoLAB <239841157+Corax-CoLAB@users.noreply.github.com>

freqtrade/exchange/exchange.py

@@ -2732,6 +2732,9 @@ class Exchange:
         """
         input_coroutines: list[Coroutine[Any, Any, OHLCVResponse]] = []
         cached_pairs = []
+        if not pair_list:
+            return input_coroutines, cached_pairs
+
         # optimization: cache timeframes as a set outside the loop to avoid repeated property access
         # and linear search
         available_timeframes = set(self.timeframes)