Don't mark empty strings for translation

In the po format, the empty string ("") is reserved and has a special use. It is always used as the msgid and key for the po file header, and has the po file header as its translation in a po file. As such, marking empty strings for translation will not work as expected, as the result returned by the gettext () call will be the entire po file header. The solution is to simply not mark empty strings for translation.