Fix bug in make_worker_proxies.py where fetching an old IP failed

make_worker_proxies.pyhad a bug in a special case where the IP look-up for a worker proxy failed, but the IP was available in the previous worker_proxies.json file. Due to a type mismatch, the script terminated in this case. The bug only occurred when running with an existing worker_proxies.json file that contained the IP for this particular worker proxy, which happened to be comet.sdsc.edu.

The fix was tested and verified with the particular worker_proxies.json file that triggered the bug.