scripts/setlocalversion: fix a bug when LOCALVERSION is empty

The commit 042da426 ("scripts/setlocalversion: simplify the short
version part") reduces indentation. Unfortunately, it also changes behavior
in a subtle way - if the user has empty "LOCALVERSION" variable, the plus
sign is appended to the kernel version. It wasn't appended before.

This patch reverts to the old behavior - we append the plus sign only if
the LOCALVERSION variable is not set.

Fixes: 042da426

 ("scripts/setlocalversion: simplify the short version part")
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>