[String customization] Never return undefined

The default `translate` function will be used if a call to the custom one returns undefined.
This is a fallback to always display something, except if the custom returns an explicit empty string.

This is useful if you want to customize only a few translation keys without redefining it via JSON `JSONEditor.defaults.languages.*` for each language, which can be pretty annoying. And it allows better synergy with other translation mechanisms / libraries.