dracut: don't skip zero-length string outfile argument

When '' was passed as outfile, dracut generated name with following
pattern:

  /boot/initramfs-${kernel}.img

With commit 486a1b9324 dracut skips ''
argument.

dracut

@@ -237,8 +237,9 @@ while (($# > 0)); do
                        ;;
         -*) printf "\nUnknown option: %s\n\n" "$1" >&2; usage; exit 1;;
         *) 
-            if ! [[ $outfile ]]; then
+            if ! [[ $outfile_set ]]; then
                 outfile=$1
+                outfile_set=y
             elif [[ $kernel = "unset" ]]; then
                 kernel=$1
             else