strfmon: EINVAL if the '+' flag and both signs are empty

According to the Open Group Base Specifications Issue 8[1], strfmon(3)
should return EINVAL when the '+' flag was included in a conversion
specification and the locale's positive_sign and negative_sign values
would both be returned by localeconv(3) as empty strings.

Austin Group Defect 1199[2] is applied, adding the [EINVAL] error.

[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/strfmon.html
[2]: https://www.austingroupbugs.net/view.php?id=1199

Reviewed by: kib
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D53912