Fix SSH key existence check logic

Author: kristapsk

tools/finalize-nakamochi.sh

@@ -125,7 +125,7 @@ run_main()
         exit 1
     fi
 
-    if [[ -z "$SSH_AUTHORIZED_KEY" ]] && [[ ! -f "$SSH_AUTHORIZED_KEY" ]]; then
+    if [[ -n "$SSH_AUTHORIZED_KEY" ]] && [[ ! -f "$SSH_AUTHORIZED_KEY" ]]; then
         echo "Error: SSH authorized pubkey file $SSH_AUTHORIZED_KEY does not exist."
         exit 1
     fi