post-receive: fix warning in script for example when pushing deleted tags

the "new" variable would be zero:
	remote: fatal: bad object 0000000000000000000000000000000000000000

NOTE: when pushing new tags the "old" variable would be zero, this was
      already fixed.

example_post-receive.sh

@@ -37,6 +37,7 @@ cd "${dir}" || exit 1
 force=0
 while read -r old new ref; do
 	test "${old}" = "0000000000000000000000000000000000000000" && continue
+	test "${new}" = "0000000000000000000000000000000000000000" && continue
 
 	hasrevs=$(git rev-list "${old}" "^${new}" | sed 1q)
 	if test -n "${hasrevs}"; then