BUG: formatPartPath silently maps empty path to '1'

[andrinoff]

Describe the bug

fetcher/fetcher.go:224-227:

func formatPartPath(path []int) string {
    if len(path) == 0 {
        return "1"
    }
    ...

Returning "1" for an empty path masks bugs in the caller (a Walk that produced no path is almost certainly wrong) and conflicts with parsePartID, which treats "" and "TEXT" as nil and "1" as a real path of length 1.

Expected behavior

Return "" (matching parsePartID's nil-path convention) and let the caller decide.