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
```

api4/handlers.go

@@ -214,7 +214,7 @@ func minimumProfessionalLicense(f handlerFunc) handlerFunc {
 	return func(c *Context, w http.ResponseWriter, r *http.Request) {
 		lic := c.App.Srv().License()
 		if lic == nil || (lic.SkuShortName != model.LicenseShortSkuProfessional && lic.SkuShortName != model.LicenseShortSkuEnterprise) {
-			c.Err = model.NewAppError("", "api.license_error.professional_or_enterprise", nil, "", http.StatusNotImplemented)
+			c.Err = model.NewAppError("", model.NoTranslation, nil, "license is neither professional nor enterprise", http.StatusNotImplemented)
 			return
 		}
 		f(c, w, r)
@@ -224,7 +224,7 @@ func minimumProfessionalLicense(f handlerFunc) handlerFunc {
 func rejectGuests(f handlerFunc) handlerFunc {
 	return func(c *Context, w http.ResponseWriter, r *http.Request) {
 		if c.AppContext.Session().Props[model.SessionPropIsGuest] == "true" {
-			c.Err = model.NewAppError("", "api.authorization_error.guest", nil, "", http.StatusNotImplemented)
+			c.Err = model.NewAppError("", model.NoTranslation, nil, "insufficient permissions as a guest user", http.StatusNotImplemented)
 			return
 		}
 		f(c, w, r)

api4/user.go

@@ -3255,7 +3255,7 @@ func updateReadStateAllThreadsByUser(c *Context, w http.ResponseWriter, r *http.
 
 func getUsersWithInvalidEmails(c *Context, w http.ResponseWriter, r *http.Request) {
 	if *c.App.Config().TeamSettings.EnableOpenServer {
-		c.Err = model.NewAppError("GetUsersWithInvalidEmails", "api.users.invalid_emails.enable_open_server.app_error", nil, "", http.StatusBadRequest)
+		c.Err = model.NewAppError("GetUsersWithInvalidEmails", model.NoTranslation, nil, "TeamSettings.EnableOpenServer is enabled", http.StatusBadRequest)
 		return
 	}