Fix minor bug in aggregate

[dlcole3]

I found that the code below gave an error when I called aggregate. It appeared to be caused by this line because the objective function being passed was zero and so was not attached to any specific node. I added a second OR statement so that it also allows for the objective to be zero. In addition, I added a @warn statement to the aggregate function so that, if the graph has no objective function but the nodes do, the user is warned that the objectives will be lost in the aggregation.

MWE:

using JuMP, Plasmo, HiGHS
g = OptiGraph()
@optinode(g, nodes[1:4])

for node in nodes
    @variable(node, 0 <= x)
    @objective(node, Min, x)
end
@linkconstraint(g, nodes[1][:x] + nodes[2][:x] >= 1)
@linkconstraint(g, nodes[2][:x] + nodes[3][:x] >= 1)
@linkconstraint(g, nodes[3][:x] + nodes[4][:x] >= 1)
@linkconstraint(g, nodes[4][:x] + nodes[1][:x] >= 1)

node_membership_vector = [1, 1, 2, 2]
partition = Plasmo.Partition(g, node_membership_vector)

apply_partition!(g, partition)
#set_to_node_objectives(g)

agg_graph, agg_map = aggregate(g)