Unify node and edge label behaviour

[diegozea]

Maybe it could be great to unify the node and edge label behavior. At this moment, names don't skip nothing and throw an error with missing. For example:

using Graphs, Plots, GraphRecipes
graph = smallgraph(:karate)
begin
	node_labels = Union{Nothing, String}[]
	for node in vertices(graph)
		if node == 1
			push!(node_labels, "Mr. Hi")
		elseif node == 34
			push!(node_labels, "John A")
		else
			push!(node_labels, nothing)
		end
	end
end
graphplot(graph, names=node_labels)