MM-46577: Remove empty translation strings (#20876)

We replace the empty translation key with <untranslated>
which is ignored by the translation check tool.

https://mattermost.atlassian.net/browse/MM-46577

```release-note
NONE
```

app/post.go

@@ -1959,12 +1959,12 @@ func (a *App) SetPostReminder(postID, userID string, targetTime int64) *model.Ap
 	}
 	err := a.Srv().Store.Post().SetPostReminder(reminder)
 	if err != nil {
-		return model.NewAppError("SetPostReminder", "app.post_reminder.app_error", nil, "", http.StatusInternalServerError).Wrap(err)
+		return model.NewAppError("SetPostReminder", model.NoTranslation, nil, "", http.StatusInternalServerError).Wrap(err)
 	}
 
 	metadata, err := a.Srv().Store.Post().GetPostReminderMetadata(postID)
 	if err != nil {
-		return model.NewAppError("SetPostReminder", "app.post_reminder.app_error", nil, "", http.StatusInternalServerError).Wrap(err)
+		return model.NewAppError("SetPostReminder", model.NoTranslation, nil, "", http.StatusInternalServerError).Wrap(err)
 	}
 
 	parsed := time.Unix(targetTime, 0).UTC().Format(time.RFC822)