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 is applied, adding the [EINVAL] error.
[1]: https://pubs.opengroup.org/onlinepubs/9799919799/functions/strfmon.html