Correctly import "service@" GSS host-based name

Correctly import "service@" GSS host-based name

The intended way to specify only a service in a GSS host-based name is
to omit the "@" separator.  Some applications include the separator
but no hostname, and this happened to yield wildcard hostname behavior
prior to commit 996353767fe8afa7f67a3b5b465e4d70e18bad7c when
shortname qualification was added.  To restore this behavior, check in
parse_hostbased() that at least one character is present after the "@"
separator before copying the hostname.  Add a test case to t_gssapi.py.

ticket: 8892
tags: pullup
target_version: 1.18-next