fix(lifecycle): support resources without a setup method

addon/-private/resources/lifecycle.ts

@@ -70,7 +70,10 @@ function setupInstance(
   let instance = new Class(owner, args);
 
   associateDestroyableChild(cache, instance);
-  instance.setup();
+
+  if ('setup' in instance) {
+    instance.setup();
+  }
 
   if ('teardown' in instance) {
     registerDestructor(instance, () => instance.teardown());

tests/unit/use-resource-test.ts

@@ -86,6 +86,34 @@ module('useResource', function () {
 
         assert.verifySteps(['setup', 'update', 'update', 'teardown']);
       });
+
+      test('setup is optional', async function (assert) {
+        class Doubler extends LifecycleResource<{ positional: [number] }> {
+          get num() {
+            return this.args.positional[0] * 2;
+          }
+        }
+
+        class Test {
+          @tracked count = 0;
+
+          data = useResource(this, Doubler, () => [this.count]);
+        }
+
+        let foo = new Test();
+
+        assert.equal(foo.data.num, 0);
+
+        foo.count = 3;
+        await settled();
+
+        assert.equal(foo.data.num, 6);
+
+        foo.count = 4;
+        await settled();
+
+        assert.equal(foo.data.num, 8);
+      });
     });
 
     module('in templates', function (hooks) {