fix(plugin-legacy): chunk may not exist (#3886)

Author: y1d7ng

packages/plugin-legacy/index.js

@@ -303,6 +303,7 @@ function viteLegacyPlugin(options = {}) {
     },
 
     transformIndexHtml(html, { chunk }) {
+      if (!chunk) return
       if (chunk.fileName.includes('-legacy')) {
         // The legacy bundle is built first, and its index.html isn't actually
         // emitted. Here we simply record its corresponding legacy chunk.