fix: handle None timezone_offset in timestamp formatting

[saschabuehrle]

Fixes #205

Timing.format_timestamp() accepted Optional[int] for timezone_offset, but it always instantiated TimeZone(self.timezone_offset). When the offset was None, this crashed in timedelta(hours=None).

This keeps the existing timezone path for integer offsets and uses plain datetime.fromtimestamp() when offset is None. Added a regression test for the None case.

Greetings, saschabuehrle

[KnugiHK]

Hi, just checking, did the fix in #206 not solve it on your end? Happy to dig back into it if it's still acting up.

[saschabuehrle]

Yeah, #206 fixes one path, but this PR covers another failing case where timezone_offset can still be None in imports. I hit that on a converted export and this guard prevents the timedelta(None) crash there too.\n\nGreetings, saschabuehrle

[KnugiHK]

I ran your test_timing.py against the current dev branch and it's passing, likely because the None guard we added in #206 to Timing.__init__ effectively handles this. Could you double-check if you are still seeing failures on the latest dev branch?