refactor: crash in timestamp formatting when timezone_offset is none

In `Timing.format_timestamp`, if `self.timezone_offset` is `None` (which is explicitly allowed by the `Optional[int]` type hint), it instantiates `TimeZone(None)`. 
When `datetime.fromtimestamp()` calls the `utcoffset()` method on this timezone object, it executes `timedelta(hours=self.offset)`, which evaluates to `timedelta(hours=None)`. This raises a `TypeError: unsupported type for timedelta hours component: NoneType`, causing the application to crash during export.


Affected files: data_model.py

Signed-off-by: Tang Vu <vuminhtang2212@gmail.com>

Whatsapp_Chat_Exporter/data_model.py

@@ -30,7 +30,8 @@ class Timing:
         """
         if timestamp is not None:
             timestamp = timestamp / 1000 if timestamp > 9999999999 else timestamp
-            return datetime.fromtimestamp(timestamp, TimeZone(self.timezone_offset)).strftime(format)
+            tz = TimeZone(self.timezone_offset) if self.timezone_offset is not None else None
+            return datetime.fromtimestamp(timestamp, tz).strftime(format)
         return None