Repro for scope with no declarations (already fixed on this stack)

Repro from T180504728 which reproduced internally and on playground, neither of 
which have #2687 yet. That PR (earlier in this stack) already fixes the issue, 
so i'm just adding the repro to help prevent regressions.

compiler/packages/babel-plugin-react-forget/src/ReactiveScopes/CodegenReactiveFunction.ts

@@ -504,7 +504,7 @@ function codegenReactiveScope(
   if (testCondition === null) {
     CompilerError.invariant(firstOutputIndex !== null, {
       reason: `Expected scope to have at least one declaration`,
-      description: `Scope '@{scope.id} has no declarations`,
+      description: `Scope '@${scope.id}' has no declarations`,
       loc: null,
       suggestions: null,
     });

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/repro-no-declarations-in-reactive-scope-with-early-return.expect.md

@@ -0,0 +1,121 @@
+
+## Input
+
+```javascript
+// @enableAssumeHooksFollowRulesOfReact @enableTransitivelyFreezeFunctionExpressions
+function Component() {
+  const items = useItems();
+  const filteredItems = useMemo(
+    () =>
+      items.filter(([item]) => {
+        return item.name != null;
+      }),
+    [item]
+  );
+
+  if (filteredItems.length === 0) {
+    // note: this must return nested JSX to create the right scope
+    // shape that causes no declarations to be emitted
+    return (
+      <div>
+        <span />
+      </div>
+    );
+  }
+
+  return (
+    <>
+      {filteredItems.map(([item]) => (
+        <Stringify item={item} />
+      ))}
+    </>
+  );
+}
+
+```
+
+## Code
+
+```javascript
+import { unstable_useMemoCache as useMemoCache } from "react"; // @enableAssumeHooksFollowRulesOfReact @enableTransitivelyFreezeFunctionExpressions
+function Component() {
+  const $ = useMemoCache(9);
+  const items = useItems();
+  let t0;
+  let t1;
+  let t2;
+  if ($[0] !== items) {
+    t2 = Symbol.for("react.early_return_sentinel");
+    bb14: {
+      let t3;
+      if ($[4] === Symbol.for("react.memo_cache_sentinel")) {
+        t3 = (t4) => {
+          const [item] = t4;
+          return item.name != null;
+        };
+        $[4] = t3;
+      } else {
+        t3 = $[4];
+      }
+      t0 = items.filter(t3);
+      const filteredItems = t0;
+      if (filteredItems.length === 0) {
+        let t4;
+        if ($[5] === Symbol.for("react.memo_cache_sentinel")) {
+          t4 = (
+            <div>
+              <span />
+            </div>
+          );
+          $[5] = t4;
+        } else {
+          t4 = $[5];
+        }
+        t2 = t4;
+        break bb14;
+      }
+      let t4;
+      if ($[6] === Symbol.for("react.memo_cache_sentinel")) {
+        t4 = (t5) => {
+          const [item_0] = t5;
+          return <Stringify item={item_0} />;
+        };
+        $[6] = t4;
+      } else {
+        t4 = $[6];
+      }
+      t1 = filteredItems.map(t4);
+    }
+    $[0] = items;
+    $[1] = t1;
+    $[2] = t2;
+    $[3] = t0;
+  } else {
+    t1 = $[1];
+    t2 = $[2];
+    t0 = $[3];
+  }
+  if (t2 !== Symbol.for("react.early_return_sentinel")) {
+    return t2;
+  }
+  let t3;
+  if ($[7] !== t1) {
+    t3 = <>{t1}</>;
+    $[7] = t1;
+    $[8] = t3;
+  } else {
+    t3 = $[8];
+  }
+  return t3;
+}
+
+```
+      
+### Eval output
+(kind: exception) Fixture not implemented!
+logs: ['The above error occurred in the <WrapperTestComponent> component:\n' +
+  '\n' +
+  '    at WrapperTestComponent (<project_root>/packages/snap/dist/sprout/evaluator.js:54:26)\n' +
+  '\n' +
+  'Consider adding an error boundary to your tree to customize error handling behavior.\n' +
+  'Visit https://reactjs.org/link/error-boundaries to learn more about error boundaries.']

compiler/packages/babel-plugin-react-forget/src/__tests__/fixtures/compiler/repro-no-declarations-in-reactive-scope-with-early-return.js

@@ -0,0 +1,29 @@
+// @enableAssumeHooksFollowRulesOfReact @enableTransitivelyFreezeFunctionExpressions
+function Component() {
+  const items = useItems();
+  const filteredItems = useMemo(
+    () =>
+      items.filter(([item]) => {
+        return item.name != null;
+      }),
+    [item]
+  );
+
+  if (filteredItems.length === 0) {
+    // note: this must return nested JSX to create the right scope
+    // shape that causes no declarations to be emitted
+    return (
+      <div>
+        <span />
+      </div>
+    );
+  }
+
+  return (
+    <>
+      {filteredItems.map(([item]) => (
+        <Stringify item={item} />
+      ))}
+    </>
+  );
+}