certs: Fix build error when CONFIG_MODULE_SIG_KEY is empty

Since b8c96a6b ("certs: simplify $(srctree)/ handling and remove
config_filename macro"), when CONFIG_MODULE_SIG_KEY is empty,
signing_key.x509 fails to build:

    CERT    certs/signing_key.x509
  Usage: extract-cert <source> <dest>
  make[1]: *** [certs/Makefile:78: certs/signing_key.x509] Error 2
  make: *** [Makefile:1831: certs] Error 2

Pass "" to the first argument of extract-cert to fix the build error.

Link: https://lore.kernel.org/linux-kbuild/20220120094606.2skuyb26yjlnu66q@lion.mk-sys.cz/T/#u
Fixes: b8c96a6b

 ("certs: simplify $(srctree)/ handling and remove config_filename macro")
Reported-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Michal Kubecek <mkubecek@suse.cz>