fix sender allow emptry string (#409)

Co-authored-by: G8s Bot <[email protected]>

Author: lucemia

pyowm/weatherapi25/national_weather_alert.py

@@ -30,7 +30,7 @@ class NationalWeatherAlert:
     :raises: *ValueError* if any parameter has wrong type
     """
     def __init__(self, sender, title, description, start_time, end_time, tags=None):
-        assert sender, 'Event sender name must be specified'
+        assert sender is not None, 'Event sender name must be specified'
         assert title, 'Event title must be specified'
         assert description, 'Event description must be specified'
         assert start_time, 'Event start time must be specified'