Fix: Handle nodes with null relation_name

[noel]

Summary

• Skip nodes that have no relation_name (such as dbt operations like on-run-end hooks) to prevent re.sub() from failing with "expected string or bytes-like object" error
• Add test to verify null relation_name nodes are properly skipped

Problem

When processing dbt manifests that contain operations (like on-run-end hooks), the build_nodes_with_columns() method would crash because these nodes have relation_name: null. The normalize_table_relation_name() function uses re.sub() which requires a string input.

Example node that caused the error:

{
  "unique_id": "operation.balboa.balboa-on-run-end-0",
  "resource_type": "model",
  "relation_name": null,
  "config": {"materialized": "view"}
}

Solution

Add a check to skip nodes without relation_name (unless they're ephemeral models, which construct their relation name differently).

Test plan

• Added unit test test_nodes_with_null_relation_name_are_skipped
• Verified fix works with real dbt manifest containing operations

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