Raise a clear error for empty YAML files

[biefan]

Summary

• reject empty YAML inputs in YamlLoadable.from_yaml_file() with a clear ValueError
• add regression coverage for both SeedDataset.from_yaml_file() and ConfigurationLoader.from_yaml_file()

Problem

yaml.safe_load() returns None for empty or whitespace-only YAML files. YamlLoadable.from_yaml_file() currently forwards that None into downstream constructors, which surfaces as internal AttributeError/TypeError exceptions depending on the caller.

Two real call paths affected today are SeedDataset.from_yaml_file() and ConfigurationLoader.from_yaml_file(), both of which currently fail with low-level exceptions instead of a clear validation error.

Testing

• .venv/bin/pytest tests/unit/models/test_seed.py tests/unit/setup/test_configuration_loader.py -q