Improve null checks for SalesOrder.shipment

[SchrodingersGat]

• Fixes small bug introduced in https://github.com/inventree/InvenTree/pull/8445

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.37%. Comparing base (0872bea) to head (cfa4da4). Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/backend/InvenTree/order/serializers.py	66.66%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #8469 +/- ## ======================================= Coverage 84.37% 84.37% ======================================= Files 1175 1175 Lines 53662 53662 Branches 2012 2012 ======================================= + Hits 45275 45276 +1 + Misses 7872 7871 -1 Partials 515 515 ``` | [Flag](https://app.codecov.io/gh/inventree/InvenTree/pull/8469/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=inventree) | Coverage Δ | | |---|---|---| | [backend](https://app.codecov.io/gh/inventree/InvenTree/pull/8469/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=inventree) | `85.86% <66.66%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=inventree#carryforward-flags-in-the-pull-request-comment) to find out more.

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