Merged PR 291: #2357 - Allow empty nodes, links, and agents arrays in config file

## Summary
Allowed the config to not have nodes, links and agents and still be parsed

## Test process
Tested with config file that doesn't have any simulation key/value pair

## Checklist
- [X] PR is linked to a **work item**
- [X] **acceptance criteria** of linked ticket are met
- [X] performed **self-review** of the code
- [X] written **tests** for any new functionality added with this PR
- [ ] updated the **documentation** if this PR changes or adds functionality
- [ ] written/updated **design docs** if this PR implements new functionality
- [ ] updated the **change log**
- [X] ran **pre-commit** checks for code style
- [ ] attended to any **TO-DOs** left in the code

#2357 - Allowed the config to not have nodes, links and agents and still be parsed

Related work items: #2357

src/primaite/game/game.py

@@ -222,8 +222,12 @@ class PrimaiteGame:
         sim = game.simulation
         net = sim.network
 
-        nodes_cfg = cfg["simulation"]["network"]["nodes"]
-        links_cfg = cfg["simulation"]["network"]["links"]
+        simulation_config = cfg.get("simulation", {})
+        network_config = simulation_config.get("network", {})
+
+        nodes_cfg = network_config.get("nodes", [])
+        links_cfg = network_config.get("links", [])
+
         for node_cfg in nodes_cfg:
             node_ref = node_cfg["ref"]
             n_type = node_cfg["type"]
@@ -390,7 +394,7 @@ class PrimaiteGame:
             game.ref_map_links[link_cfg["ref"]] = new_link.uuid
 
         # 3. create agents
-        agents_cfg = cfg["agents"]
+        agents_cfg = cfg.get("agents", [])
 
         for agent_cfg in agents_cfg:
             agent_ref = agent_cfg["ref"]  # noqa: F841
@@ -439,12 +443,12 @@ class PrimaiteGame:
                     agent_settings=agent_settings,
                 )
             else:
-                msg(f"Configuration error: {agent_type} is not a valid agent type.")
+                msg = f"Configuration error: {agent_type} is not a valid agent type."
                 _LOGGER.error(msg)
                 raise ValueError(msg)
             game.agents[agent_cfg["ref"]] = new_agent
 
         # Set the NMNE capture config
-        set_nmne_config(cfg["simulation"]["network"].get("nmne_config", {}))
+        set_nmne_config(network_config.get("nmne_config", {}))
 
         return game