vue 2.5.17版本 render函数不支持返回字符串

2.5.17版本render函数不再支持直接返回字符串，必须返回数组或vnode节点，如果返回字符串的话，渲染为空。详情可见源码。

function createFunctionalComponent (
  Ctor,
  propsData,
  data,
  contextVm,
  children
) {
  var options = Ctor.options;
  var props = {};
  var propOptions = options.props;
  if (isDef(propOptions)) {
    for (var key in propOptions) {
      props[key] = validateProp(key, propOptions, propsData || emptyObject);
    }
  } else {
    if (isDef(data.attrs)) { mergeProps(props, data.attrs); }
    if (isDef(data.props)) { mergeProps(props, data.props); }
  }

  var renderContext = new FunctionalRenderContext(
    data,
    props,
    children,
    contextVm,
    Ctor
  );

  var vnode = options.render.call(null, renderContext._c, renderContext);
　 // 此处判断如果为vnode或数组，才返回
  if (vnode instanceof VNode) {
    return cloneAndMarkFunctionalResult(vnode, data, renderContext.parent, options)
  } else if (Array.isArray(vnode)) {
    var vnodes = normalizeChildren(vnode) || [];
    var res = new Array(vnodes.length);
    for (var i = 0; i < vnodes.length; i++) {
      res[i] = cloneAndMarkFunctionalResult(vnodes[i], data, renderContext.parent, options);
    }
    return res
  }
}