git: treat "git -C '<path>'" as a no-op when <path> is empty

git: treat "git -C '<path>'" as a no-op when <path> is empty

'git -C ""' unhelpfully dies with error "Cannot change to ''",
whereas the shell treats `cd ""' as a no-op.  Taking the shell's
behavior as a precedent, teach git to treat `-C ""' as a no-op, as
well.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>