Why is the output result empty?

[devenliu]

input default.less

@import '~antd/lib/style/themes/default.less';

const less = require('less');
const NpmImportPlugin = require('less-plugin-npm-import');
const options = {
  plugins: [ 
    new NpmImportPlugin({ prefix: '~' }),
  ]
}
less.render(content, options).then(result => result.css);

output default.css

/* stylelint-disable at-rule-empty-line-before,at-rule-name-space-after,at-rule-no-unknown */
/* stylelint-disable no-duplicate-selectors */
/* stylelint-disable */
/* stylelint-disable declaration-bang-space-before,no-duplicate-selectors,string-no-newline */