scripts/download.pl: fix downloads with wget

scripts/download.pl: fix downloads with wget

Several users of wget for downloads (curl is not available in the
system) have reported broken download functionality:

 wget --tries=5 --timeout=20 --output-document=-  https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.10.142.tar.xz
 http://: Invalid host name.

Thats all happening due to '' was passed as an argument, which got later
expanded to http://.

In the context of a list constructor '' is not nothing, it is an empty
string element.  So fix it by using () as it will yield "nothing" and
thus not introduce an empty string element.

Fixes: #10692
Fixes: 90c6e3aedf16 ("scripts: always check certificates")
Signed-off-by: Jo-Philipp Wich <jo@mein.io> [shellwords() -> ()]
Signed-off-by: Petr Štetiar <ynezz@true.cz>