Fix calling parse_pathspec with no paths nor PATHSPEC_PREFER_* flags

When parse_pathspec() is called with no paths, the behavior could be
either return no paths, or return one path that is cwd. Some commands
do the former, some the latter. parse_pathspec() itself does not make
either the default and requires the caller to specify either flag if
it may run into this situation.

I've grep'd through all parse_pathspec() call sites. Some pass
neither, but those are guaranteed never pass empty path to
parse_pathspec(). There are two call sites that may pass empty path
and are fixed with this patch.

[jc: added a test from Antoine's bug report]

Reported-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>