Use correct year for generated birthdays events

'X-APPLE-OMIT-YEAR' is not always present, at least iOS 12.4 uses the hard coded date of 1604 (the start of the gregorian calendar) when the year is unknown.

cf. https://forums.bitfire.at/topic/2050/bday-without-year-for-vcard-3-0-if-server-drops-property-x-apple-omit-year and https://gitlab.com/CardBook/CardBook/-/issues/586



Signed-off-by: Sven Strickroth <email@cs-ware.de>