help_print_desc: Ensure newline is printed for empty description.

Callers expect help_print_desc to always end on a new line, so that the
next option can be printed at the start of a new line.  Currently,
however, this function does nothing for an empty description string,
leading to incorrect output unless the option name was long enough
that help_print_optstring took care of it.

Add a special case to fix that up, and a new test case.