6737819 tests some unusual behavior whereby the nonProxyHosts might need to be set to "" empty string in order
to force proxying for all destinations including localhost. That might make sense (as 6737819 suggests) if the proxy is located on the same machine (for testing purposes) as both the client and server.
We broke this behavior in 7141335 by always adding the default proxy bypass list.
The solution here is to continue the behavior introduced by 7141335, except in the case where the proxy
property is set, but its value is an empty string. In that case (only), no bypass list will be set,
and the proxy will be used for all destinations.