Config file parser crashes on empty values

## Bug Report\n\nWhen a TOML config file has empty string values, the config parser crashes with a TypeError.\n\n```toml\n[acme]\napi_key = ""\nendpoint = "https://ingest.acme-sdk.dev"\n```\n\n```\nTypeError: cannot parse '' as float\n```\n\nThe issue is in `_interpolate_env_vars` — it tries to type-cast empty strings.