Fixed issue with module state when extending the module Store

[ResuBaka]

Related issues

closes #

Short description and why it's useful

The first one was that an null value in an state was changed to an object.
The second one was that now an null value was not changed to an object we
need to change the null to an object when we try to change the type the
original state.

Here some examples:

// Old code:

state: {
  current: null
  hello: null
}

// State from extend module: 

state: {
  hello: { hello: 'world' }
}

// After extend
state: {
  current: {}
  hello: { hello: 'world' }
}

// New code:

state: {
  current: null
  hello: null
}

// State from extend module: 

state: {
  hello: { hello: 'world' }
}

// After extend
state: {
  current: null
  hello: { hello: 'world' }
}

Screenshots of visual changes before/after (if there are any)

Which environment this relates to

Check your case. In case of any doubts please read about Release Cycle

• Test version (https://test.storefrontcloud.io) - this is a new feature or improvement for Vue Storefront. I've created branch from develop branch and want to merge it back to develop
• RC version (https://next.storefrontcloud.io) - this is a stabilisation fix for Release Candidate of Vue Storefront. I've created branch from release branch and want to merge it back to release
• Stable version (https://demo.storefrontcloud.io) - this is an important fix for current stable version. I've created branch from hotfix or master branch and want to merge it back to hotfix

Upgrade Notes and Changelog

• No upgrade steps required (100% backward compatibility and no breaking changes)
• I've updated the Upgrade notes and Changelog on how to port existing VS sites with this new feature

IMPORTANT NOTICE - Remember to update CHANGELOG.md with description of your change

Contribution and currently important rules acceptance

• I read and followed contribution rules

[ResuBaka]

I think we should write some test cases around that logic as there may be some other cases where it could break.
So we should create an issue for writing some test.

[pkarw]

Makes sense for me; thanks!

[pspaczek]

looks good, thanks @ResuBaka