fix: Enabled sub folder, fetching wrong static JSON URL

[ics-nishihara]

🐛 The bug
I enable sub folder, fetching static JSON URL is wrong.

🛠️ To reproduce
Steps to reproduce the behavior:

1. setting sub folder at nuxt.config.json

router: {
  base: "/foo/",
}

2. using useFetch() method normaly.
3. access the page, the fetching static JSON is wrong. Nuxt tries fetching "fooKEY.json" not "foo/KEY.json"

🌈 Expected behaviour
Fetching the correct URL; "foo/KEY.json".

ℹ️ Additional context
It caused by here. When does not set sub folder settings, the options.publicPath indicates / but /foo/ sets is indicates /foo. Fetching "fooKEY.json" because of the slash is dropped.

Sorry for my poor English