Fix Core panicking on a zero-value Version

[xianjianlf2]

Description

Calling Core() on a zero-value Version panics because its segment slice is empty. Return a new version representing 0.0.0, consistent with the zero value's String and Equal behavior.

The regression test checks the returned version and verifies that the receiver remains unchanged.

Related Issue

No linked issue.

How Has This Been Tested?

The regression test reproduces the panic before the fix.

Passing after the fix:

• go test -race -count=1 ./...
• go vet ./...
• git diff --check

Code, tests, and this draft were prepared with OpenAI Codex.

[hashicorp-cla-app]

All committers have signed the CLA.

[tgross]

LGTM but needs rebase

[tgross]

LGTM

But FWIW I hate that this Equal comparison works: https://go.dev/play/p/wG8ub8LEjK8

package main

import (
	"fmt"

	"github.com/hashicorp/go-version"
)

func main() {
	v1 := &version.Version{}
	v2 := version.Must(version.NewVersion("0.0.0"))
	fmt.Println("v1:", v1.String())
	fmt.Println("v2:", v2.String())
	fmt.Println(v1.Equal(v2))
	fmt.Println(v1.Core())
}

approved