Fix null and non submodules objects properties in state

[aymasse]

Hello,

I'm writing this PR to fix two issues I encountered with the current version (2.0.2) of this library.

The first is that the function isFieldASubModule in submodule.ts was trying to access a property from state properties initialized to null because in JS typeof null is strangely equal to 'object'.

The second one was a bit more tricky, as all object based state properties were all set to {}, even if they should have been Set or Map.

Feel free to tell me if this PR needs more work.

[tinou98]

I faced the same second problem, but in the end I'm not sure that Set or Map are allowed in a Vuex state :

From https://vuex.vuejs.org/guide/state.html

The data you store in Vuex follows the same rules as the data in a Vue instance, ie the state object must be plain. See also: Vue#data.

From Vue#data

The data object for the Vue instance. Vue will recursively convert its properties into getter/setters to make it “reactive”. The object must be plain: native objects such as browser API objects and prototype properties are ignored. A rule of thumb is that data should just be data - it is not recommended to observe objects with their own stateful behavior.

The only case where this fix is required is for Array (for which I made a pull request #39)

[aymasse]

Oh I did not know about this rule, although it makes sense now that I see it. I'll let @michaelolof decide if this rule should be strictly enforced or not.

[danielroe]

I think null is a legitimate value for a store state.