[cfid-458] Make ScimGroupJsonSerializer include only non-null and non-empty fields.

Usually this is achieved by simply adding a 'include = JsonSerialize.Inclusion.NON_NULL'
setting, but since we switched to using custom (de)serializers for the SCIM objects, we
need to take care of this ourselves. Fixes [#41838481].

Change-Id: I2c3aeb5d9b53662bf65299d7c8d73f368ddf03c5