Mark group as omitempty
Mark group as omitempty

Because it's optional, and we don't want to distinguish between it being
not present, and it having been set to empty.

Signed-off-by: Lorenzo Manacorda <86995a091f8a1385082bd10715048b52ee386a6a@kinvolk.io>

kinvolk/habitat-operator,kinvolk/habitat-operator