cfg80211: make certificate generation more robust

If all net/wireless/certs/*.hex files are deleted, the build
will hang at this point since the 'cat' command will have no
arguments. Do "echo | cat - ..." so that even if the "..."
part is empty, the whole thing won't hang.

Cc: stable@vger.kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Link: https://lore.kernel.org/r/iwlwifi.20210618133832.c989056c3664.Ic3b77531d00b30b26dcd69c64e55ae2f60c3f31e@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>