Deserialize a null list field as None instead of raising (#131)

[CedricConday]

Fixes #131

Some Xero API responses return null for a field typed as an array. For example payroll_au_api.reject_leave_application(...) responds with:

{"Id": "...", "Status": "OK", "LeaveApplications": null}

Deserializing this raises:

TypeError: 'NoneType' object is not iterable

because deserialize_list does [deserialize(...) for sub_data in data] without guarding against data is None.

Change

Return None early when data is None, faithfully representing the null field. This mirrors deserialize_model, which already special-cases a None payload.

def deserialize_list(data_type, data, model_finder):
    if data is None:
        return None
    ...

Test

Added test_deserialize_list_none. It fails on master (TypeError) and passes with the fix. Full test_api_client suite (148 tests) stays green.