fix: fix nullable type in ts 2.6

[HerringtonDarkholme]

Fix #181

TS 2.6 seems to change Object.getOwnPropertyDescriptor

Note key is already the property name, there is no need to check null in runtime.

[ktsn]

@HerringtonDarkholme Thanks! But npm run example still produce compile error as reported in the original thread like:

Type '<VC extends VueClass<Vue>>(target: VC) => VC' is not assignable to type 'typeof App'

[HerringtonDarkholme]

That is another problem in Vuex's type definition.