fix: (lingui/core) i18n error if id is undefined

packages/core/src/i18n.test.ts

@@ -290,6 +290,23 @@ describe("I18n", () => {
     expect(handler).toHaveBeenCalledTimes(2)
   })
 
+  it("._ should emit missing event for undefined id", () => {
+    const i18n = setupI18n({
+      locale: "en",
+      messages: { en: {} },
+    })
+
+    const handler = jest.fn()
+    i18n.on("missing", handler)
+    // @ts-expect-error 'id' should be of 'MessageDescriptor' or 'string' type.
+    i18n._()
+    expect(handler).toHaveBeenCalledTimes(1)
+    expect(handler).toHaveBeenCalledWith({
+      id: "",
+      locale: "en",
+    })
+  })
+
   describe("params.missing - handling missing translations", () => {
     it("._ should return custom string for missing translations", () => {
       const i18n = setupI18n({

packages/core/src/i18n.ts

@@ -213,6 +213,11 @@ export class I18n extends EventEmitter<Events> {
     options?: MessageOptions
   ): string {
     let message = options?.message
+
+    if (!id) {
+      id = ""
+    }
+
     if (!isString(id)) {
       values = id.values || values
       message = id.message