I swear this was there at some point. Honest!
I’m only seeing Empty now as well.
This seems to take care of the user created properties. Replace the content: "" part with whatever you like.
.metadata-property .metadata-input-longtext:empty:before {
content: "👻🕷️🎃";
color: var(--text-faint);
}

When you enter a value, whatever the content is, is removed.

Obsidian’s default Properties are a bit different, and using similar for those produces some unwanted results:
.metadata-property .multi-select-input:empty:before {
content: "abcd";
}

I’ll have another look at it later if I have time.
I’ll move this to the CSS category at this point and we can whittle away at it.