~ in path resolved into empty string if home_dir is unknown (or unavailable)

[micolous]

Something I noticed while writing #105, but I opted to leave things as they are:

When expanding a ~, if home_dir (or env_home_dir) returns None (eg: $HOME is unset, or on a platform like WASM where there is no home directory), then tilde_expansion() turns it into an empty PathBuf by unwrap_or_default():

which-rs/src/finder.rs

Lines 268 to 276 in 1b5f7be

	if o == "~" {
	let mut new_path = home_dir().unwrap_or_default();
	new_path.extend(component_iter);
	#[cfg(feature = "tracing")]
	tracing::trace!(
	"found tilde, substituting in user's home directory to get {}",
	new_path.display()
	);
	Cow::Owned(new_path)

That would cause which to search a path relative to the current working directory for binaries, rather than the intended path.

This should probably return some error instead (or skip searching paths containing ~ if there is no home directory), but I think that could be a significant behaviour change on some platforms / environments.