returning nothing make the app bug silently

Hello,

I'm using nuxt which is using vue-meta with a head object.

If you do the following, this break interface silently : 

```js
head() {
    return;
}
```

instead you have to do the following

```js
head() {
    return null;
}

// OR

head() {
    return {};
}
```

I think add a check for this and dispatch at least a console warning could be nice.

Regards :)