feat: add logic to skip empty array columns when no columns or --skip-add-source-columns

[LuigiCerone]

Problem

_sync_doc_section in sync_operations.py always wrote the columns key unconditionally:

doc_section["columns"] = incoming_columns

When a node has no columns (e.g. a source with --skip-add-source-columns enabled), incoming_columns is [] and the result is columns: [] being written to every source table osmosis touches — even though the user explicitly opted out of column management.

The two behaviors were fully decoupled: --skip-add-source-columns prevented osmosis from querying the database for columns, but the empty list was still written unconditionally by _sync_doc_section regardless.

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Fix

Only write the columns key when there are actual columns to document; remove it otherwise:

if incoming_columns:
    doc_section["columns"] = incoming_columns
else:
    doc_section.pop("columns", None)

This is safe — dbt treats a missing columns key and columns: [] identically.

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Tests

Two unit tests added to test_sync_operations.py, using lightweight mocks (no DuckDB fixture needed):

Test	Description
test_sync_doc_section_no_columns_key_not_added	Verifies columns is never added when the node has no columns
test_sync_doc_section_existing_empty_columns_removed	Verifies a pre-existing columns: [] is cleaned up on the next run