Panic on empty trait impl

[mohammadfawaz]

The script below should not compile but with an error complaining that the function f is not implemented for S.

script;

trait A {
    fn f(self) -> u64;
}
struct S {
    x: u64,
}
impl A for S { 
}
fn main() {
}

Instead, the compiler panics with:

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', sway-core/src/parse_tree/declaration/impl_trait.rs:80:48
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

We do emit the correct error when the trait has multiple functions and only emit a subset of them in the impl block. The above seems to only be a problem when the impl block is empty.