fix: using useFetch breaks raise undefined error on 404 page

[farnabaz]

🐛 The bug
When using useFetch composable inside setup function will raise undefined error when navigating to 404 page.

I think this might be related to generating fetchKey

🛠️ To reproduce
This issue only happens in static website, Steps to reproduce the behavior:

1. Clone https://github.com/nuxt/nuxtjs.org/tree/32f6498640d0cb99772a70fc8cf27477351ad767
2. yarn generate
3. yarn start
4. Go to /404 page and check web console

🌈 Expected behaviour

Evaluating fetch hook without raising an error.

[kilobyte2007]

This is an issue for me too, any progress here?

Are there plans to fix this in the 2.*
I might be willing to help with a PR if someone points me in the right direction.