feat: Add empty constructor for GNNHeteroGraph

[askorupka]

Adding empty constructor for GNNHeteroGraph so that g = GNNHeteroGraph() is working. This fixes #338.

g = GNNHeteroGraph()
GNNHeteroGraph:
  num_nodes: Dict()
  num_edges: Dict()

[CarloLucibello]

Thanks, looks good. The coverage has decreased because tests should be added to "test/GNNGraphs/gnnheterograph.jl". The test could be something like this

g = GNNHeterograph()
@test isempty(g.num_nodes)
g = add_edges(g, (:user, :like, :actor) => ([1,2,3,3,3], [3,5,1,9,4]))
@test g.num_nodes[:user] == 3
@test g.num_nodes[:actor] == 9
@test g.num_edges[(:user, :like, :actor)] == 5
...