Bug Report: panic in GetTrackings when Pagination is nil

[xmatzx]

Library version:
github.com/AfterShip/tracking-sdk-go v9.0.0

Issue:
When calling GetTrackings, the SDK panics if the API response does not include a Pagination object.

In get_trackings.go, inside the Execute method, Pagination is accessed directly without checking if it is nil, which results in:
panic: runtime error: invalid memory address or nil pointer dereference

Steps to Reproduce:

1. Use the GetTrackings request.
2. Receive a response where Pagination is missing or empty.
3. Observe that the SDK crashes with a panic.

Expected Behavior:
The SDK should handle cases where Pagination is nil gracefully (e.g., return nil or a default Pagination object) instead of panicking.