DynamoDB: update_item() can now remove last item from set

[bblommers]

Fixes #8128

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 94.49%. Comparing base (0b0f29f) to head (9ac9ade). Report is 3 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #8129 +/- ## ======================================= Coverage 94.49% 94.49% ======================================= Files 1150 1150 Lines 99021 99025 +4 ======================================= + Hits 93567 93571 +4 Misses 5454 5454 ``` | [Flag](https://app.codecov.io/gh/getmoto/moto/pull/8129/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getmoto) | Coverage Δ | | |---|---|---| | [servertests](https://app.codecov.io/gh/getmoto/moto/pull/8129/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getmoto) | `28.81% <25.00%> (-0.01%)` | :arrow_down: | | [unittests](https://app.codecov.io/gh/getmoto/moto/pull/8129/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=getmoto) | `94.46% <100.00%> (+<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=getmoto#carryforward-flags-in-the-pull-request-comment) to find out more.

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[github-actions[bot]]

This is now part of moto >= 5.0.15.dev19