function normalizeQuotes(
  ast: PostCSSValueAST,
  _: mixed,
): PostCSSValueAST {
  ast.walk((node) => {
    if (node.type !== 'string') {
      return;
    }
    if (node.value === '') {
      node.quote = '"';
    }
  });
  return ast;
}
Make empty strings use consistent double quotes