Add unit tests for NetworkXStorage

[NumberChiffre]

Description

This PR is to add unit tests strictly for NetworkXStorage data class, as well as some minor adjustments.

Some details to keep in mind:

• Self-loop and parallel edges are enabled.
• Community schema has default hyperparameters not configurable by users.
• Only a single level for the community schema right after Leiden clustering because the Leiden algorithm is creating a single-level clustering (overwriting any previous multi-level clustering data that might have existed).

Updates

• Add handling of non-existent node for node and edge degree.
• Add unit tests.