BUG: registry.ParseReference returns false positive when registry is empty

[qweeah]

Below code is expected to panic but can parse the invalid reference /oras

package main

import (
	"fmt"

	_ "crypto/sha256"

	"oras.land/oras-go/v2/registry"
)

func main() {
	rawRef := "/oras"
	ref, err := registry.ParseReference(rawRef)
	if err != nil {
		panic(err)
	}

	fmt.Println("Registry:", ref.Registry)
	fmt.Println("Repository:", ref.Repository)
} 

Version: oras.land/oras-go/v2 v2.4.0

The root cause is url.ParseRequestURI allows empty domain name, ORAS-go should enforce the non-empty checking.