Fix arbitrary "additional" table when not specified.

[MrLixm]

On the instancing.data.arbitrary attribute. If the 5th column for the additional table is not specified an error is raised where something is excepting a table.
Reproduce the error again and fix it.

[MrLixm]

Error line151 for hierarchical :

arbtr_data = arbtr_data["additional"]  -- type: table
for addit_target, addit_value in pairs(arbtr_data) do
    self:add(addit_target, addit_value)
end

arbtr_data was just not expected to be nil