fix: preserve empty result error messages in JSON - #125

An error with an empty message changes into a successful result after a JSON round trip:

data, _ := json.Marshal(mo.Err[int](errors.New("")))
var result mo.Result[int]
json.Unmarshal(data, &result)
// result.IsOk() == true

Decode the message as a pointer so a present empty string still selects the error branch. This also preserves error precedence when the JSON contains both result and an empty error message.

Both regression cases fail before the fix. go test ./... -count=1, go test -race ./... -count=1, go vet ./..., gofmt and the changed-file diff check pass. The race suite ran on Windows with Clang and cgo enabled.