fix: Fix Backlinks not applying the display class (#519)

* fix: Fix `Backlinks` not applying the display class

Fix #518

* fix: Apply `displayClass` to all layout components

* refactor: Use same style

* fix: Remove `undefined` class using coalescing operator

quartz/components/Spacer.tsx

@@ -1,8 +1,7 @@
 import { QuartzComponentConstructor, QuartzComponentProps } from "./types"
 
 function Spacer({ displayClass }: QuartzComponentProps) {
-  const className = displayClass ? `spacer ${displayClass}` : "spacer"
-  return <div class={className}></div>
+  return <div class={`spacer ${displayClass ?? ""}`}></div>
 }
 
 export default (() => Spacer) satisfies QuartzComponentConstructor