[BUGFIX beta] Don't rely on non-cached reference

This commit fixes a bug where `OutletComponentReference`
inadvertently relied on broken behavior: it returned `null`, assuming
that that would cause the reference to be rebuilt. But
glimmerjs/glimmer-vm#337 fixed the underlying bug, causing the reference to
be properly cached. As a result, we need to remember that we rendered a
`null` component, so that swapping back to the original component
replaces the empty component with the rendered component.

packages/ember-glimmer/lib/syntax/outlet.js

@@ -114,7 +114,6 @@ class OutletComponentReference {
   value() {
     let { outletNameRef, parentOutletStateRef, definition, lastState } = this;
 
-
     let outletName = outletNameRef.value();
     let outletStateRef = parentOutletStateRef.get('outlets').get(outletName);
     let newState = this.lastState = outletStateRef.value();
@@ -130,7 +129,7 @@ class OutletComponentReference {
     } else if (hasTemplate) {
       return this.definition = new OutletComponentDefinition(outletName, newState.render.template);
     } else {
-      return null;
+      return this.definition = null;
     }
   }
 }