Differentiate between "No Key" and "No Values" while deserialising/unmarshalling a Yaml list to Golang struct

I am unmarshalling a Yaml config file to Golang struct. I want to implement following logic:

if blacklist key is not there in yaml:
	then allow everything
else if blacklist key is there but there are no values:
	then block everything
else if blacklist values are there in yaml:
	then filter out only the listed items

I'm not able to differentiate between the last two scenarios.
Essentially both look same i.e. "there is no value for blacklist key" but I'm wondering if there's any way possible. (without introducing any additional flag in yaml).
I tried with pointer type but it doesn't work with that.
Here's the simplified code sample:
https://play.golang.org/p/UheBEPFhzsg

package main

import (
	"fmt"
	"gopkg.in/yaml.v2"
)

type Config struct {
	Host string        `yaml:"host"`
	Blacklist []string `yaml:"blacklist"`
}

func main() {
	configDataWithBlacklistValues := `
host: localhost
blacklist: 
  - try
  - experiment
`
	configDataWithoutBlacklistValues := `
host: localhost
blacklist:
`
	configDataWithoutBlacklistKey := `
host: localhost
`

	var configWithBlacklistValues Config    // this config should filter out blacklist items
	var configWithoutBlacklistValues Config // this config should filter out everything (no blacklist values = everything to blacklist)
	var configWithoutBlacklistKey Config    // this config should filter out nothing (no blacklist key = nothing to blacklist)

	yaml.Unmarshal(([]byte)(configDataWithBlacklistValues), &configWithBlacklistValues)
	yaml.Unmarshal(([]byte)(configDataWithoutBlacklistValues), &configWithoutBlacklistValues)
	yaml.Unmarshal(([]byte)(configDataWithoutBlacklistKey), &configWithoutBlacklistKey)

	fmt.Printf("%+v\n", configWithBlacklistValues)
	fmt.Printf("%+v\n", configWithoutBlacklistValues)
	fmt.Printf("%+v\n", configWithoutBlacklistKey)
	
	/*
	if blacklist key is not there in yaml:
		then allow everything
	else if blacklist key is there but there are no values:
		then block everything
	else if blacklist values are there in yaml:
		then filter out only the listed items
	*/
}

Codesample with list as pointer type.
https://play.golang.org/p/wK8i3dLCHWQ

type HostList []string

type Config struct {
	Host string `yaml:"host"`
	Blacklist *HostList `yaml:"blacklist"`
}

A simple workaround for your solution, would be to implement the pointer type HostList, but encode the 2) case YAML, i.e. data without blacklist values as below

host: localhost
blacklist: []

By doing so, your unmarshal will return a zero length slice ([]string{}) and not a nil slice. So your code can check for nil slice for the 3rd case alone.

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