Fix item swap working on backpacks

[Intybyte]

Related Issues (if applicable)

Resolves #4250

Checklist

• [x] I have fully tested the proposed changes and promise that they will not break everything into chaos.
• [ ] I have also tested the proposed changes in combination with various popular addons and can confirm my changes do not break them.
• [ ] I have made sure that the proposed changes do not break compatibility across the supported Minecraft versions (1.16.* - 1.20.*).
• [x] I followed the existing code standards and didn't mess up the formatting.
• [ ] I did my best to add documentation to any public classes or methods I added.
• [ ] I have added Nonnull and Nullable annotations to my methods to indicate their behaviour for null values
• [ ] I added sufficient Unit Tests to cover my code.

[github-actions[bot]]

Your Pull Request was automatically labelled as: "✨ Fix" Thank you for contributing to this project! ❤️

[github-actions[bot]]

Slimefun preview build

A Slimefun preview build is available for testing! Commit: feacb42b

https://preview-builds.walshy.dev/download/Slimefun/4275/feacb42b

Note: This is not a supported build and is only here for the purposes of testing. Do not run this on a live server and do not report bugs anywhere but this PR!

[RaphaelFakhri]

Clever!