Preserve url better in exportTrailingSlash (#13840)

Discovered while doing https://github.com/vercel/next.js/pull/13333. `#` or `?` used to be stripped if there wasn't a value behind

packages/next/next-server/lib/router/rewrite-url-for-export.ts

@@ -7,7 +7,7 @@ export function rewriteUrlForNextExport(url: string): string {
     // Append a trailing slash if this path does not have an extension
     if (!/\.[^/]+\/?$/.test(path)) path += `/`
   }
-  if (qs) path += '?' + qs
-  if (hash) path += '#' + hash
+  if (typeof qs === 'string') path += '?' + qs
+  if (typeof hash === 'string') path += '#' + hash
   return path
 }

test/integration/export/next.config.js

@@ -17,6 +17,8 @@ module.exports = (phase) => {
         '/about': { page: '/about' },
         '/button-link': { page: '/button-link' },
         '/hash-link': { page: '/hash-link' },
+        '/empty-hash-link': { page: '/empty-hash-link' },
+        '/empty-query-link': { page: '/empty-query-link' },
         '/get-initial-props-with-no-query': {
           page: '/get-initial-props-with-no-query',
         },

test/integration/export/pages/empty-hash-link.js

+import Link from 'next/link'
+
+export default () => (
+  <div id="empty-hash-link-page">
+    <Link href="/hello#">
+      <a id="empty-hash-link">Empty Hash link</a>
+    </Link>
+  </div>
+)

test/integration/export/pages/empty-query-link.js

+import Link from 'next/link'
+
+export default () => (
+  <div id="empty-query-link-page">
+    <Link href="/hello?">
+      <a id="empty-query-link">Empty Query link</a>
+    </Link>
+  </div>
+)

test/integration/export/test/browser.js

@@ -28,6 +28,24 @@ export default function (context) {
       expect(link).toMatch(/\/hash-link\/#hash$/)
     })
 
+    it('should preserve hash symbol on empty hash Link', async () => {
+      const browser = await webdriver(context.port, '/empty-hash-link')
+      const link = await browser
+        .elementByCss('#empty-hash-link')
+        .getAttribute('href')
+
+      expect(link).toMatch(/\/hello\/#$/)
+    })
+
+    it('should preserve question mark on empty query Link', async () => {
+      const browser = await webdriver(context.port, '/empty-query-link')
+      const link = await browser
+        .elementByCss('#empty-query-link')
+        .getAttribute('href')
+
+      expect(link).toMatch(/\/hello\/\?$/)
+    })
+
     it('should not add trailing slash on Link when disabled', async () => {
       const browser = await webdriver(context.portNoTrailSlash, '/')
       const link = await browser