Add empty path capture docs and tests #3782

Job	Run time
check-docs	1m 5s
cargo-hack	9m 18s
cargo-public-api-crates (axum)	1m 35s
cargo-public-api-crates (axum-core)	1m 25s
cargo-public-api-crates (axum-extra)	1m 37s
cargo-public-api-crates (axum-macros)	1m 15s
check	3m 31s
cargo-deny check (advisories)	2m 2s
cargo-deny check (bans licenses sources)	2m 6s
dependencies-are-sorted	1m 23s
test-nightly	4m 37s
test-msrv	8m 4s
armv5te-unknown-linux-musleabi	2m 19s
test-docs	5m 15s
test-versions (stable)	7m 39s
test-versions (beta)	7m 57s
wasm32-unknown-unknown	33s
	1h 1m 41s

Provide feedback