Fix IndexError when visualizung subgraphs with zerod edges or nodes

[sudhansu-24]

Describe your changes

guards wmg.visualise.plot_2d from crashing with IndexError when attempting to apply attribute coloring to empty graph components (zero edges or zero nodes).

when splitting graphs via filter_graph or split_graph_by_edge_attribute, users often process a dictionary of subgraphs. If a subgraph contains nodes but no edges (or is completely empty), plotting it with an edge_color_attr used to crash inside _get_graph_attr_values because it indexed [0] on an empty list

This PR fixes it by:

1. Adding early-exit guards in nx_draw_with_pos_and_attr to gracefully skip coloring when number_of_edges() == 0 or number_of_nodes() == 0.
2. Adding a descriptive ValueError inside _get_graph_attr_values for when it is called directly on empty graph components.
3. Adding 4 regression tests to test_graph_plots.py covering these edge cases.

Issue Link

solves #130

Type of change

• 🐛 Bug fix (non-breaking change that fixes an issue)
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• 📖 Documentation (Addition or improvements to documentation)

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