UI: Fix missing unassigned icon in System theme

[shiina424]

Description

The unassigned icon is missing in the System theme.
This issue occurs in version 31.0-beta1 or later, but does not occur in version 30.2.3.
this issue was caused by this change. (maybe.) cb02696
I'm not sure if this change is correct, so please feel free to close it if it's not.

Before

Before.mp4

After

After.mp4

Motivation and Context

I want the icons to be displayed correctly.
I’m not sure how many people are using the system theme, but I’ll fix it (even though I don’t use it myself).

How Has This Been Tested?

I checked with the local build.
I am not sure if the code change is correct, but the icon is now displaying.

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.