Empty styles or selectors cause error

Currently If I write the following test it fails:

```js
it("compiles empty styles", (done) => {
assert.strictEqual(
plugin('p { }', {
compileEnv,
}),
"p { }"
);
});
```

We have, for various reasons, a number of `css.resolve``;` calls in our code which result in the following errors being thrown during compilation.

```
[styled-jsx-plugin-postcss] did not compile the following CSS:
at Array.forEach ()
at Array.forEach ()
at Array.forEach ()
```

I've started digging into this issue a little bit and it looks as if https://github.com/giuseppeg/styled-jsx-plugin-postcss/blob/master/processor.js#L18 never returns so the compilation times out. The only reason I'm thinking it's a bug here is because if I do `postcss().process('p {}', { from: false })` it returns back `p {}` without issue.

I've started debugging this myself, but wanted to document the behavior in an issue in case there's some backstory on this.