This fixes an issue where when the engine looked up groups (for example,
in ABM node names), NodeDefManager's m_group_to_items would contain nodes
with a group value of zero, resulting in nodes with flammable = 0 being
burned by a fire mod with a group:flammable checking ABM.
It brings consistency to the behaviour described in the api
documentation, where zero and nil groups should be the same.
4d7f296717