Changes

SPIGOT-7637: Bad logic in checking nullability of AttributeModifier slots

Also fix Class loading order issues, which resulted in EquipmentSlot#getGroup returning null, since EquipmentSlot and EquipmentSlotGroup referencing each other on class init.
This caused EquipmentSlot to being init first, when the fields in EquipmentSlotGroup are still null, resulting in the group being null.