fixes #324

the new XML serialization code emitted empty optional fields that would
lead to parse errors with keepass and keepassxc.

the ISO8601 timestamp format used for KDBX4 serialization was rejected by KeePass,
which expects timestamps to be encoded as Base64 integers.

Co-authored-by: Michael <94733164+SatelliteDish@users.noreply.github.com>