Resolve dry-run fails when version scripts exists and version table not exists yet

[PavelPawlowski]

Resolves #326

Addresses issue

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/root/.schema_change/lib/python3.12/site-packages/schemachange/cli.py", line 75, in <module>
    main()
  File "/root/.schema_change/lib/python3.12/site-packages/schemachange/cli.py", line 71, in main
    deploy(config=config, session=session)
  File "/root/.schema_change/lib/python3.12/site-packages/schemachange/deploy.py", line 102, in deploy
    script_metadata = versioned_scripts.get(script.name)
                      ^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'get'

Which occurs in dry-run mode when version scripts are being deployed and version table does not exists yet.

Instead of None, returns empty dictionary defaultdict(dict) as change_history.