allows for empty `toml::toml!` invocations

[sanbox-irl]

I don't know too much about macros, so perhaps this is too noisy a change, but this simply allows calling toml::toml!{} without anything inside it, creating a blank toml::Value::Table.

In unit tests, it can read more clearly for an empty toml document when other tests nearby are constructing real data, and it has a very small footprint, so it doesn't seem like an issue in general.