kbuild: deb-pkg: Don't fail if modules.order is missing

Kernels built without CONFIG_MODULES might still want to create -dbg deb
packages but install_linux_image_dbg() assumes modules.order always
exists. This obviously isn't true if no modules were built, so we should
skip reading modules.order in that case.

Fixes: 16c36f88 ("kbuild: deb-pkg: use build ID instead of debug link for dbg package")
Signed-off-by: Matt Fleming <mfleming@cloudflare.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>