DEV: Fix message section link filters displaying icons (#18484)

Follow-up to c0037dc0f0

app/assets/javascripts/discourse/app/lib/sidebar/user/messages-section/message-section-link.js

@@ -73,10 +73,14 @@ export default class MessageSectionLink {
   }
 
   get prefixType() {
-    return "icon";
+    if (this._isInbox) {
+      return "icon";
+    }
   }
 
   get prefixValue() {
-    return "inbox";
+    if (this._isInbox) {
+      return "inbox";
+    }
   }
 }

app/assets/javascripts/discourse/tests/acceptance/sidebar-user-messages-section-test.js

@@ -161,6 +161,13 @@ acceptance(
           ),
           `personal message ${type} link is marked as active`
         );
+
+        assert.notOk(
+          exists(
+            `.sidebar-section-messages .sidebar-section-link-personal-messages-${type} .sidebar-section-link-prefix`
+          ),
+          `prefix is not displayed for ${type} personal message section link`
+        );
       });
     });