DEV: Add support for arrays to concatClass (#26777)

It'd be useful in cases like:

```gjs
get flagNames() {
  return this.flags?.map((flag) => flag && `flag-${flag.name}`);
  // w/o this change you'd have to handle it here:
  // return this.flags?.map((flag) => flag && `flag-${flag.name}`).filter(Boolean).join(" ");
}

<template>
  <span class={{concatClass "gadget" this.flagNames}}>
  {{! …or here: }}
  <span class={{concatClass "gadget" (concatClass this.flagNames)}}>
```

(also: replaces ember's `compact()` with `filter(Boolean)`)

app/assets/javascripts/discourse/app/helpers/concat-class.js

@@ -1,5 +1,5 @@
 export default function concatClass(...args) {
-  const classes = args.compact().join(" ");
+  const classes = args.flat().filter(Boolean).join(" ");
 
   if (classes.length) {
     return classes;

app/assets/javascripts/discourse/tests/integration/helpers/concat-class-test.js

@@ -2,7 +2,6 @@ import { render } from "@ember/test-helpers";
 import { hbs } from "ember-cli-htmlbars";
 import { assert, module, test } from "qunit";
 import { setupRenderingTest } from "discourse/tests/helpers/component-test";
-import { query } from "discourse/tests/helpers/qunit-helpers";
 
 module("Integration | Helper | concat-class", function (hooks) {
   setupRenderingTest(hooks);
@@ -10,28 +9,28 @@ module("Integration | Helper | concat-class", function (hooks) {
   test("One class given", async function () {
     await render(hbs`<button class={{concat-class "foo"}} />`);
 
-    assert.equal(query("button").className, "foo");
+    assert.dom("button").hasAttribute("class", "foo");
   });
 
   test("Multiple class given", async function () {
     this.set("bar", "bar");
     await render(hbs`<button class={{concat-class "foo" this.bar}} />`);
 
-    assert.equal(query("button").className, "foo bar");
+    assert.dom("button").hasAttribute("class", "foo bar");
   });
 
   test("One undefined class given", async function () {
     this.set("bar", null);
     await render(hbs`<button class={{concat-class "foo" this.bar}} />`);
 
-    assert.equal(query("button").className, "foo");
+    assert.dom("button").hasAttribute("class", "foo");
   });
 
   test("Only undefined class given", async function () {
     this.set("bar", null);
     await render(hbs`<button class={{concat-class null this.bar}} />`);
 
-    assert.notOk(query("button").hasAttribute("class"));
+    assert.dom("button").doesNotHaveAttribute("class");
   });
 
   test("Helpers used", async function () {
@@ -39,6 +38,14 @@ module("Integration | Helper | concat-class", function (hooks) {
       hbs`<button class={{concat-class (if true "foo") (if true "bar")}} />`
     );
 
-    assert.equal(query("button").className, "foo bar");
+    assert.dom("button").hasAttribute("class", "foo bar");
+  });
+
+  test("Arrays", async function () {
+    await render(
+      hbs`<button class={{concat-class (array) (array "foo" "bar") (array null)}} />`
+    );
+
+    assert.dom("button").hasAttribute("class", "foo bar");
   });
 });