Assert that Dependency::name is never empty, prevent 'install ""' from crashing

Assert that Dependency::name is never empty, prevent 'install ""' from crashing

An explicit `cargo install ""` would cause clap to pass an empty crate-name,
leading to a panic(). We now assert() that Dependency::name is never the
empty string and prevent the situation in the first place by not allowing
the crate-name to be empty for `install`.

Fixes #5229