bug: empty `.tree` files cause a panic

[mds1]

To reproduce:

• Create a .tree file that's empty
• Run bulloak -w ./**/*.tree
• Get the below output:

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /Users/mds/.cargo/registry/src/index.crates.io-6f17d22bba15001f/bulloak-0.4.6/src/scaffold/parser.rs:202:58
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

Bulloak should exit without panicking here, and should print an error similar to that when the contents of the .tree file are invalid