misc fixes for unloading via AIM

[marilynias]

Summary

None

Purpose of change

Follow-up to #79187, where I introduced unloading via AIM.
Clothing containing items could not be unloaded.
De-duplicate checks and error-messages already in Character::unload().

Describe the solution

sitem -> contents_count returns 0 for clothing with items in them. Replace it with correct empty check.
Be less picky what to pass to unload() and let it display the error message.

Describe alternatives you've considered

fix sitem -> contents_count, but I don't know if its intended or how it affects other parts.

Testing

Clothing can be unloaded

Additional context

[marilynias]

huh

E: Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/u/util-linux/uuid-dev_2.39.3-9ubuntu6.1_amd64.deb  404  Not Found [IP: 40.81.13.82 80]
E: Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/u/util-linux/libblkid-dev_2.39.3-9ubuntu6.1_amd64.deb  404  Not Found [IP: 40.81.13.82 80]
E: Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/u/util-linux/libmount-dev_2.39.3-9ubuntu6.1_amd64.deb  404  Not Found [IP: 40.81.13.82 80]
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

https://github.com/CleverRaven/Cataclysm-DDA/actions/runs/12947198948/job/36113432940?pr=79316#step:2:311