TT#111150 Use test -z instead of a prefixed x

This construct was used due to ancient bogus shell implementations, but
it is of no relevance anymore. Use the "modern" -z test instead.

Change-Id: Iad882c99148e548e926b083df8ca428c591e2c4f
Fixes: shellcheck SC2268

debian/ngcp-rtpengine-kernel-dkms.postinst

@@ -19,7 +19,7 @@ if [ "$VIRT" = "yes" ]; then
 	echo "Container environment detected. Skip dkms"
 else
 	isadded=$(dkms status -m "$name" -v "$version")
-	if [ "x${isadded}" = "x" ] ; then
+	if [ -z "${isadded}" ] ; then
 		dkms add -m "$name" -v "$version"
 	fi