style: update slot empty state

[RominaMarchetti]

What does this PR change?

In the backpack, when there is an empty slot we are displaying an icon which gives the sense of that the category is blocked. To fix that misleading empty visual message, this PR replaces the icon for a simple background.

BEFORE

AFTER

Test Steps

1. Launch the explorer
2. Go to the backpack, in the wearables section, and quit the item equipped on every category to check the icon is not visible anymore.
3. Go to the emotes section an repeat the instruction above.

[github-actions]

Windows and Mac build successfull in Unity Cloud! You can find a link to the downloadable artifact below.

Name	Link
Commit	0cb7e52
Logs	https://github.com/decentraland/unity-explorer/actions/runs/13317276585
Download Windows	https://github.com/decentraland/unity-explorer/suites/34365398452/artifacts/2589177317
Download Windows S3	https://explorer-artifacts.decentraland.org/@dcl/unity-explorer/branch/style/backpack-empty-slot/Decentraland_windows64.zip
Download Mac	https://github.com/decentraland/unity-explorer/suites/34365398452/artifacts/2589262368
Download Mac S3	https://explorer-artifacts.decentraland.org/@dcl/unity-explorer/branch/style/backpack-empty-slot/Decentraland_macos.zip
Built on	2025-02-13T22:39:37Z

[anicalbano]

✔️ Reviewed by QA and working as expected.
Other sections that contain images have been reviewed as regression.

14.02.2025_09.11.14_REC.mp4

14.02.2025_09.13.58_REC.mp4