Add failing test for #14978

(cherry picked from commit 793d123)

packages/@ember/-internals/glimmer/tests/integration/content-test.js

@@ -818,6 +818,42 @@ moduleFor(
 
       this.assertContent('<b>Max</b><i>James</i>');
     }
+
+    ['@test empty content in trusted curlies [GH#14978]']() {
+      this.render('before {{{value}}} after', {
+        value: 'hello',
+      });
+
+      this.assertContent('before hello after');
+
+      this.runTask(() => this.rerender());
+
+      this.assertStableRerender();
+
+      this.runTask(() => set(this.context, 'value', undefined));
+
+      this.assertContent('before  after');
+
+      this.runTask(() => set(this.context, 'value', 'hello'));
+
+      this.assertContent('before hello after');
+
+      this.runTask(() => set(this.context, 'value', null));
+
+      this.assertContent('before  after');
+
+      this.runTask(() => set(this.context, 'value', 'hello'));
+
+      this.assertContent('before hello after');
+
+      this.runTask(() => set(this.context, 'value', ''));
+
+      this.assertContent('before  after');
+
+      this.runTask(() => set(this.context, 'value', 'hello'));
+
+      this.assertContent('before hello after');
+    }
   }
 );