DEV: Ensure getter can always return without error (#18621)

Normally, arguments passed to components are lazily evaluated. `get prefixElementColors` will only be evaluated for `@prefixType="span"`. However, when using the Ember Inspector in development, arguments are eagerly evaluated and their values displayed in the inspector. Therefore we need to make sure that getters can always be evaluated without exceptions being thrown.

app/assets/javascripts/discourse/app/components/sidebar/section-link.js

@@ -52,6 +52,10 @@ export default class SectionLink extends Component {
   }
 
   get prefixElementColors() {
+    if (!this.args.prefixElementColors) {
+      return;
+    }
+
     const prefixElementColors = this.args.prefixElementColors.filter((color) =>
       color?.slice(0, 6)
     );