fix(vue2): handling un-namespaced vuex module

[tsukkee]

I found Vue Devtools 6 fails to handle Vuex modules without namespace (namespaced = false).
The getters of that kind of modules seems to be got from module.context.getters.

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I fixed the part of formatStoreForInspectorState and it is called from following part.

https://github.com/vuejs/vue-devtools/blob/d5e0579b35a7c93869ddb839e3863219202ab0ec/packages/app-backend-vue2/src/plugin.ts#L101-L114

This means getters argument of formatStoreForInspectorState is store._makeLocalGettersCache, but store._makeLocalGettersCache doesn't look includes root modules' ones and un -namespaced moduels' ones but only includes namespaced modules' ones.

So before my fixing, TypeError: Cannot convert undefined or null to object error occurs at line 350 since getters of un-namespaced modules are not contained by getters[path].

https://github.com/vuejs/vue-devtools/blob/d5e0579b35a7c93869ddb839e3863219202ab0ec/packages/app-backend-vue2/src/plugin.ts#L348-L351