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

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

  /boot/initramfs-$kernel

With commit 486a1b9324d6fc7bc534d8147d64b8b259692c46 dracut skips ''
argument.

diff --git a/dracut b/dracut
index 6b67dc0..c16b222 100755 (executable)
--- a/dracut
+++ b/dracut
@@ -237,7 +237,7 @@ while (($# > 0)); do
                        ;;
         -*) printf "\nUnknown option: %s\n\n" "$1" >&2; usage; exit 1;;
         *) 
-            if ! [[ $outfile ]]; then
+            if ! [[ ${outfile+x} ]]; then
                 outfile=$1
             elif [[ $kernel = "unset" ]]; then
                 kernel=$1