fallback to empty string message "" when get nil value

i18n/message.go

@@ -188,7 +188,7 @@ func stringSubmap(k string, v interface{}, strdata map[string]string) error {
 func isMessage(v interface{}) bool {
 	reservedKeys := []string{"id", "description", "hash", "leftdelim", "rightdelim", "zero", "one", "two", "few", "many", "other"}
 	switch data := v.(type) {
-	case string:
+	case nil, string:
 		return true
 	case map[string]interface{}:
 		for _, key := range reservedKeys {

i18n/parse.go

@@ -112,6 +112,13 @@
 			messages = append(messages, childMessages...)
 		}
 
+	case nil:
+		if isInitialCall {
+			return nil, errInvalidTranslationFile
+		}
+		m, err := NewMessage("")
+		return []*Message{m}, err
+
 	default:
 		return nil, fmt.Errorf("unsupported file format %T", raw)
 	}

i18n/parse_test.go

@@ -156,6 +156,37 @@ outer:
 				}},
 			},
 		},
+		{
+			name: "YAML empty key test",
+			file: `
+some-keys:
+    non-empty-key: not empty
+    empty-key-but-type-specified: ""
+    empty-key:
+    null-key: null`,
+			path:           "en.yaml",
+			unmarshalFuncs: map[string]UnmarshalFunc{"yaml": yaml.Unmarshal},
+			messageFile: &MessageFile{
+				Path:   "en.yaml",
+				Tag:    language.English,
+				Format: "yaml",
+				Messages: []*Message{
+					{
+						ID:    "some-keys.non-empty-key",
+						Other: "not empty",
+					},
+					{
+						ID: "some-keys.empty-key-but-type-specified",
+					},
+					{
+						ID: "some-keys.empty-key",
+					},
+					{
+						ID: "some-keys.null-key",
+					},
+				},
+			},
+		},
 	}
 	for _, testCase := range testCases {
 		t.Run(testCase.name, func(t *testing.T) {