set_msgstr(None) segfaults with gettext >= 0.18.2

set_msgstr(None) tries to set the msgstr to NULL, despite po_message_set_msgstr being documented as taking a string, with "" being used to represent the lack of a translation. This works fine in old gettexts, as po_message_set_msgstr is a no-op if the value hasn't changed. However, gettext 0.18.2 switches the default strings in a message from NULL to "", so po_message_set_msgstr(msg, NULL) is no longer a no-op, and it segfaults.

pygettextpo should translate None to "" instead of NULL.