FIX: Don't crash when MF definitions are missing

Currently, if MF definitions are missing (typically because there’s a
compilation error), `I18n.messageFormat` will try to access
`I18n._mfMessages.hasMessage` resulting in a crash that will in turn
crash Ember.

This patch addresses the issue by using the optional chaining operator
making the `I18n.messageFormat` method return a "Missing Key" message.
MF strings won’t be rendered properly, but the site will stay usable.

app/assets/javascripts/discourse-i18n/src/index.js

@@ -379,7 +379,7 @@ export class I18n {
   }
 
   messageFormat(key, options) {
-    const message = this._mfMessages.hasMessage(
+    const message = this._mfMessages?.hasMessage(
       key,
       this._mfMessages.locale,
       this._mfMessages.defaultLocale

app/assets/javascripts/discourse/tests/unit/lib/i18n-test.js

@@ -12,6 +12,7 @@ module("Unit | Utility | i18n", function (hooks) {
     this._translations = I18n.translations;
     this._extras = I18n.extras;
     this._pluralizationRules = { ...I18n.pluralizationRules };
+    this._mfMessages = I18n._mfMessages;
 
     I18n.locale = "fr";
 
@@ -109,6 +110,7 @@ module("Unit | Utility | i18n", function (hooks) {
     I18n.translations = this._translations;
     I18n.extras = this._extras;
     I18n.pluralizationRules = this._pluralizationRules;
+    I18n._mfMessages = this._mfMessages;
   });
 
   test("defaults", function (assert) {
@@ -356,4 +358,19 @@ module("Unit | Utility | i18n", function (hooks) {
       );
     });
   });
+
+  test("messageFormat", function (assert) {
+    assert.ok(
+      I18n.messageFormat("posts_likes_MF", { count: 2, ratio: "high" }).match(
+        /2 replies/
+      ),
+      "It works properly"
+    );
+    I18n._mfMessages = null;
+    assert.strictEqual(
+      I18n.messageFormat("posts_likes_MF", { count: 2, ratio: "high" }),
+      "Missing Key: posts_likes_MF",
+      "Degrades gracefully if MF definitions are not available."
+    );
+  });
 });