read on nil config works (nil-go#189)

config.go

@@ -87,6 +87,10 @@ func (c *Config) Load(loader Loader) error {
 // and decodes it into the given object pointed to by target.
 // The path is case-insensitive.
 func (c *Config) Unmarshal(path string, target any) error {
+	if c == nil {
+		return nil
+	}
+
 	c.nocopy.Check()
 
 	decodeHook := c.decodeHook
@@ -129,6 +133,10 @@ func (c *Config) split(key string) []string {
 // from loaders for the given path. It blur sensitive information.
 // The path is case-insensitive.
 func (c *Config) Explain(path string) string {
+	if c == nil {
+		return path + " has no configuration.\n\n"
+	}
+
 	c.nocopy.Check()
 
 	explanation := &strings.Builder{}

config_test.go

@@ -14,6 +14,16 @@ import (
 	"github.com/nil-go/konf/provider/env"
 )
 
+func TestConfig_nil(t *testing.T) {
+	var config *konf.Config
+
+	assert.True(t, !config.Exists([]string{"key"}))
+	assert.Equal(t, "key has no configuration.\n\n", config.Explain("key"))
+	var value string
+	assert.NoError(t, config.Unmarshal("key", &value))
+	assert.Equal(t, "", value)
+}
+
 func TestConfig_Load(t *testing.T) {
 	t.Parallel()
 

provider.go

@@ -31,6 +31,10 @@ type Watcher interface {
 //
 // It's used by the loader to check if the configuration has been set by other loaders.
 func (c *Config) Exists(path []string) bool {
+	if c == nil {
+		return false
+	}
+
 	c.nocopy.Check()
 
 	return maps.Sub(c.values, path) != nil