fix(seeder): use cast_dict for seed.variables to ensure non-None dict

Agent-Logs-Url: https://github.com/ds-wizard/engine-tools/sessions/42b27cab-0c05-490d-b21a-674a802556c3

Co-authored-by: MarekSuchanek <9638527+MarekSuchanek@users.noreply.github.com>

Author: Copilot

packages/dsw-data-seeder/dsw/data_seeder/config.py

@@ -5,8 +5,8 @@
     ConfigKey,
     ConfigKeys,
     ConfigKeysContainer,
+    cast_dict,
     cast_int,
-    cast_optional_dict,
     cast_optional_int,
     cast_str,
 )
@@ -32,7 +32,7 @@ class _SeedKeys(ConfigKeysContainer):
         yaml_path=['seed', 'variables'],
         var_names=[],
         default=None,
-        cast=cast_optional_dict,
+        cast=cast_dict,
     )