fix(install): adjust this.$options.router judgment

[jectychen]

The judgment "isDef(this.$options.router)" can be more rigorous.
Since in some cases, the router can be null, which will cause the execute error.
And We have faced this situation in @testing-library/vue/render.js
BTW, this change have been seen in the vue-router-next.

[posva]

Thanks! can you provide a simple reproduction of when this would fail? Or more context maybe? Why would the statement need to check for more than false values?

[posva]

Closing due to inactivity. Please open a new issue with a reference to this one if you can follow up with more information.

[gaaamii]

I met same problem with testing-library/vue@5, then I could avoid this issue with below.

render(MyComponent, { router: undefined }) // pass router option as undefined explicitly

I think this PR does not have to be merged because this problem occured in testing-library/vue@5 which is for EOL version of Vue, Vue@2.

But I comment for someone who uses same version as me. I would be happy if it is helpful to someone 😺