fix: avoid invalid modules with definePage query

Fix #338

Author: posva

playground/src/pages/index.vue

@@ -1,3 +1,11 @@
+<script lang="ts" setup>
+definePage({
+  meta: {
+    title: 'hello',
+  },
+})
+</script>
+
 <template>
   <main>
     <h1>Home</h1>

src/core/definePage.ts

@@ -31,16 +31,18 @@ export function definePageTransform({
   code: string
   id: string
 }): Thenable<TransformResult> {
-  if (!code.includes(MACRO_DEFINE_PAGE)) return
+  // are we extracting only the definePage object
+  const isExtractingDefinePage = MACRO_DEFINE_PAGE_QUERY.test(id)
+
+  if (!code.includes(MACRO_DEFINE_PAGE)) {
+    return isExtractingDefinePage ? 'export default {}' : undefined
+  }
 
   // TODO: handle also non SFC
 
   const sfc = parseSFC(code, id)
   if (!sfc.scriptSetup) return
 
-  // are we extracting only the definePage object
-  const isExtractingDefinePage = MACRO_DEFINE_PAGE_QUERY.test(id)
-
   const { script, scriptSetup, getSetupAst } = sfc
   const setupAst = getSetupAst()