Missing string values should be exported as null rather than an empty string

[spiffyjr]

Fallbacks for ngx-translate don't consider an empty string as undefined (rightfully so, imo). I prose changing empty strings to export as null rather than "".

[pbrejdak]

Is this can be merged? It's really annoying because using "" instead of null causing displaying literally nothing, however with null it's display value to translate which is better in my thinking, at least add it as param to extract command.

[biesbjerg]

Closed with 97e8937