Identity domain structs missing json omitempty parameter

[F21]

This issue relates to oracle/terraform-provider-oci#1812

The structs in the identitydomains where mandatory:"false" are missing the json:"omitempty" tag.
As an example here is a non-exhaustive list of structs where the omitempt tag is missing:

• https://github.com/oracle/oci-go-sdk/blob/master/identitydomains/user_name.go#L54
• https://github.com/oracle/oci-go-sdk/blob/master/identitydomains/user_groups.go#L45

This results in those fields being sent to the server as an empty string and since the server expects them to have a minimum length of 1, the operation fails.