Rollup merge of #148495 - ChrisDenton:path_is_empty, r=workingjubilee

Implement Path::is_empty

This implements `Path::is_empty` which is simply a convenience wrapper for `path.as_os_str().is_empty()`.

Tracking issue: rust-lang/rust#148494
ACP: rust-lang/libs-team#687

Author: Zalathar