use bashism from previous commit consistently

package-deb

@@ -15,7 +15,7 @@
 
 BINDISTS=${BINDISTS:-jessie}
 
-if [ -n "$JENKINS_HOME" ]; then
+if [[ -n "${JENKINS_HOME}" ]]; then
 	set -x
 	# Handle old chroot naming on our jenkins servers. Used in the bottom.
 	chroot_postfix="-sbuild";
@@ -53,7 +53,7 @@ ln ../../$SOURCE ../varnish_$V.$MINOR.orig.tar.gz
 sed -i -e "s|^Section: \([^/]*\)\$|Section: varnish-$V/\1|" debian/control
 
 # Take version override set on Jenkins builds into account.
-if [ -n "$DEBVERSION" ]; then
+if [[ -n "${DEBVERSION}" ]]; then
 	FULL_VERSION="$V.$MINOR-$DEBVERSION"
 	dch -v "$FULL_VERSION" "Release build #$BUILD_NUMBER ID: $BUILD_ID"
 else