drive: fix index out of bounds when parsing request local paths (#15517)

Fix the index out of bound panic when a request is made to the local
fileserver mux with a valid secret-token, but missing share name.

Example error:

    http: panic serving 127.0.0.1:40974: runtime error: slice bounds out of range [2:1]

Additionally, we document the edge case behavior of utilities that
this fileserver mux depends on.

Signed-off-by: Craig Hesling <craig@hesling.com>

drive/driveimpl/shared/pathutil_test.go

@@ -40,6 +40,7 @@ func TestJoin(t *testing.T) {
 		parts []string
 		want  string
 	}{
+		{[]string{}, "/"},
 		{[]string{""}, "/"},
 		{[]string{"a"}, "/a"},
 		{[]string{"/a"}, "/a"},