fix(store): initial state should not be overwritten by defaults

[markwhitfeld]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngxs/store/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[X] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: #902
There was a regression caused by PR #791 in this line:
https://github.com/ngxs/store/pull/791/files#diff-084c7b85b6562e3126b58451d5f8943aR20

This only affects Emitter plugin users. This is because of the way that Emitter inherits from Store to create an EmitStore. I found the point at which the state was being overwritten. See this stackblitz:
https://stackblitz.com/edit/ngxs-no-initial-state-debug

Ngxs does not expect a plugin to inherit from Store and essentially run this code twice. Emitter does some things with Ngxs internals that are not usual plugin behaviours.

What is the new behavior?

This code is now protected from being run twice. The default values are only written into the store if there is no current value in the store.

Does this PR introduce a breaking change?

[ ] Yes
[X] No

[markwhitfeld]

I have tested HMR in the integration app and can confirm that it is working great.