Fix convert_start_end_from_negative()

The application will malfunctioning if --start-chapter exists and --end-chapter don't (vice versa)

mangadex_downloader/utils.py

@@ -376,14 +376,9 @@ def convert_start_end_from_negative(start_num, end_num, data):
     - "--end-page"
     """
 
-    if start_num is None and end_num is None:
+    if start_num is None or end_num is None:
         return start_num, end_num
 
-    if start_num is None:
-        start_num = 0
-    if end_num is None:
-        end_num = 0
-
     if start_num >= 0 and end_num >= 0:
         return start_num, end_num