Fix type error in Child template options

Add empty scope to satisfy the ExplicitTemplateOnlyOptions overload
when neither eval nor component is provided.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: NullVoxPopuli

packages/@glimmer-workspace/integration-tests/test/keywords/fn-runtime-test.ts

@@ -59,6 +59,7 @@ class KeywordFn extends RenderTest {
 
     const Child = template('<button {{on "click" @callback}}>Click</button>', {
       strictMode: true,
+      scope: () => ({}),
     });
 
     const compiled = template('<Child @callback={{fn greet "hello"}} />', {