refactor: use default preset for cssnano

lib/builder/webpack/utils/postcss.js

@@ -47,7 +47,7 @@ export default class PostcssConfig {
           // https://cssdb.org/#staging-process
           stage: 2
         },
-        'cssnano': !this.dev
+        'cssnano': this.dev ? false : { preset: 'default' }
       }
     }
   }