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
```

model/utils.go

@@ -33,6 +33,7 @@ const (
 	NUMBERS          = "0123456789"
 	SYMBOLS          = " !\"\\#$%&'()*+,-./:;<=>?@[]^_`|~"
 	BinaryParamKey   = "MM_BINARY_PARAMETERS"
+	NoTranslation    = "<untranslated>"
 )
 
 type StringInterface map[string]any