fix: don't crash on AsTraitPath with empty path #14924

Job	Run time
build-acvm-js	1m 30s
build-nargo	2m 16s
build-noirc-abi	1m 29s
build-noir-wasm	3m 50s
noirc_abi	1m 38s
ACVM JS (Browser)	1m 35s
ACVM JS (Node.js)	38s
noir_codegen	1m 13s
Example scripts	12s
Noir JS	1m 11s
Check external repo - ${{ matrix.project.repo }}	0s
Integration Tests (Browser)	2m 27s
noir_wasm	1m 59s
Integration Tests (Node)	2m 21s
End	1s
	22m 20s

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