submodule: don't pass empty string arguments to submodule--helper clone

submodule: don't pass empty string arguments to submodule--helper clone

When --reference or --depth are unused, the current git-submodule.sh
results in empty "" arguments appended to the end of the argv array
inside git submodule--helper clone. This is not caught because the argc
count is not checked today.

Fix git-submodule.sh to only pass an argument when --reference or
--depth are used, preventing the addition of two empty string arguments
on the tail of the argv array.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Reviewed-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>