Group parsing fails when 'creation' field is missing from WhatsApp response

[tovmeod]

When calling GetJoinedGroups(), the parser fails with error "didn't find required attribute 'creation'" for groups where WhatsApp's API response doesn't include the creation timestamp field.

This is logged as a warning but actually is losing data, I can't get the group info

"Error parsing group 3682796@g.us: [didn't find required attribute 'creation']"

In group.go line 706, the parseGroupNode function uses UnixTime() which requires the field to exist:
If server response doesn't include the creation attribute in the group node, UnixTime() returns an error and parsing fails completely

Maybe it should change line 706 to use OptionalUnixTime() instead:

group.GroupCreated = ag.OptionalUnixTime("creation")