fix: initialize root state as an empty object if state function returns no value

[n7olkachev]

Just a moment ago I had hard time debugging this code:

const state = () => {}
...
new Vuex.Store({
    state,
    modules,
    ...
})

If state is undefined and you use modules, Vuex can't create store. This small change will make it easier to find this error.

[ktsn]

Thank you for your contribution. However, I think providing an empty object as a default value is more appropriate here because modules' state actually behaves so.
https://github.com/vuejs/vuex/blob/dev/src/module/module.js#L9

[n7olkachev]

Is it ok now?

[ktsn]

LGTM