fix: JsonDataException when 4PX returns 'tkTimezone':null

[gsilvan]

4PX tracking fails if 4PX API returns 'tkTimezone':null in tracks[].

Failure(com.squareup.moshi.JsonDataException: Non-null value 'tkTimezone' was null at $.data[0].tracks[7].tkTimezone)

Example: 4PX3002235116157CN (https://track.4px.com/#/result/0/4PX3002235116157CN)

{
  "tkCode": "FPX_I_RCUK",
  "tkDesc": "Released from customs: customs cleared.",
  "tkLocation": "",
  "tkTimezone": null,
  "tkDate": "2025-11-22T05:30:00.000+0000",
  "tkDateStr": "2025-11-22 13:30:00",
  "tkCategoryCode": "I",
  "tkCategoryName": "Import Clearance",
  "spTkSummary": null,
  "spTkZipCode": null,
  "tkTranslatedDesc": "Released from customs: customs cleared.",
  "tkTranslatedSummary": null,
  "sigPicUrl": null,
  "isSigPic": null
}

Proposed fix: make val tkTimezone optional.

[itsvic-dev]

lgtm