gpg: Fix exporting of zero length user ID packets.

* g10/build-packet.c (do_user_id): Avoid indeterminate length header.

We are able to import such user ids but when exporting them the
exported data could not be imported again because the parser bails out
on invalid keyrings. This is now fixed and should be backported.

Note that in 1.4 and 2.0 this is only an issue for attribute packets.
In 2.1 user IDs were also affected.a

• Signed-off-by: Werner Koch <wk@gnupg.org>