Fix generation of birthday, deathdate and anniversary in case where no year is...

Fix generation of birthday, deathdate and anniversary in case where no year is set - which is allowed as per https://tools.ietf.org/html/rfc6350#section-6.2.5

 (#26756)

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>
Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>