unmarshal: empty document results in map (#640)

Fixes #602

unmarshaler.go

@@ -174,7 +174,13 @@ func (d *decoder) FromParser(v interface{}) error {
 		return fmt.Errorf("toml: decoding pointer target cannot be nil")
 	}
 
-	err := d.fromParser(r.Elem())
+	r = r.Elem()
+	if r.Kind() == reflect.Interface && r.IsNil() {
+		newMap := map[string]interface{}{}
+		r.Set(reflect.ValueOf(newMap))
+	}
+
+	err := d.fromParser(r)
 	if err == nil {
 		return d.strict.Error(d.p.data)
 	}

unmarshaler_test.go

@@ -1800,6 +1800,16 @@ func TestIssue596(t *testing.T) {
 	require.Error(t, err)
 }
 
+func TestIssue602(t *testing.T) {
+	var v interface{}
+	err := toml.Unmarshal([]byte(""), &v)
+	require.NoError(t, err)
+
+	var expected interface{} = map[string]interface{}{}
+
+	require.Equal(t, expected, v)
+}
+
 //nolint:funlen
 func TestUnmarshalDecodeErrors(t *testing.T) {
 	examples := []struct {