Keep translation files in sync with base language (Automatically add empty translations)

### Describe the problem

We use JSON v4 translation files with English as base language. When we add a new translation key to the English JSON file, Weblate doesn't add it to other languages until a translator translates it in the Weblate UI. We treat empty strings as missing, and it would be nice for us, to always have the same JSON keys in all languages (to be able to easily diff the files, for example).

### Describe the solution you'd like

It would be great if Weblate could add keys that are in the base language file to other languages with an empty string value. Basically what the "Cleanup translation files" add-on does for removing keys that don't exist in the base language anymore, but for adding keys. Or what the "Automatic translation" add-on does, but adding an empty value instead of a machine translation.

### Describe alternatives you've considered

I have also tried to do it with Bulk Edit or Search & Replace, but didn't find a way to enter an empty string as a replacement string.

### Screenshots

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### Additional context

_No response_