parse_user_host_path, empty path should be allowed

[kircherlike]

when parse_user_host_path, empty path should be allowed

when using "sftp root@[2001::16%eth0]", the error output: ssh: Could not resolve hostname [2001::16%eth0]: Name or service not known Connection closed. Connection closed

fix sftp ipv6 login failed accidental like this: File "/root/!" not found.

[kircherlike]

@djmdjm

[djmdjm]

I can't replicate this problem - sftp 'djm@[fe80::1%lo0]' works as expected on Linux and OpenBSD