fix(testing): return [] instead of None from get_logs for non-Chrome browsers

When running tests with non-Chrome drivers, get_logs() returned None, causing
assertions like `assert dash_duo.get_logs() == []` to fail. Returning an empty
list is consistent with the Chrome path, which also returns an empty list when
there are no logs.

Author: troclaux

dash/testing/browser.py

@@ -629,10 +629,10 @@ def get_logs(self):
                 if entry["timestamp"] > self._last_ts
             ]
         warnings.warn(
-            "get_logs always return None with webdrivers other than Chrome",
+            "get_logs always return [] with webdrivers other than Chrome",
             stacklevel=2,
         )
-        return None
+        return []
 
     def reset_log_timestamp(self):
         """reset_log_timestamp only work with chrome webdriver."""