babel-plugin-server: non-identifier expression as JSX element name

`babel-plugin-server` prerenders `<undefined><undefined>` given a non-identifier expression as JSX element name.

A minimum test case:

```sh
$ echo '<Foo.Bar />;' | ./node_modules/.bin/babel --plugins ./src/transform/server
({
  __prerendered__: "dom",
  segments: ["<undefined", 1, "></undefined>"]
});
```