update_repo.sh: do not panic when a repo is empty

yum/update_repo.sh

@@ -23,7 +23,11 @@ update_repo()
 
 for repo in dom0-updates installer qubes-dom0 ; do
     echo "--> Processing repo: $repo..."
-    check_repo $repo/rpm -o $repo/repodata || continue
+    ls $repo/rpm/*.rpm 2>/dev/null
+    if [ $? -ne 0 ]; then
+        echo "Empty repo, skipping..."
+        continue
+    fi
     if [ x$NO_SIGN != x"1" ]; then
         check_repo $repo/rpm -o $repo/repodata || continue
     fi