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.