UI: Adds dark mode image for master slot empty state

[zydtiger]

Description

Adds a dark mode image for when the master course slot is empty. Makes it more coherent with dark mode UI.

Change Log

Before

After

[zydtiger]

I think my vscode formatter reformatted the file. The only line I changed was this:

[JiaqiWang18]

I think my vscode formatter reformatted the file. The only line I changed was this:

That's fine because we want the format to be consistent with rest of the codebase but TSLint checks failed so there are more format issues that need to be fixed