perfdhcp does not print all the RTT stats on no received packets

When there are no received packets, an exception is thrown in ExchangeStats::getAvgDelay() which results in min delay, avg delay displayed like so:

min delay: inf ms
avg delay: Delay summary unavailable! No packets received.

max delay, std deviation, collected packets are missing.

Either display all or display none for easier automated parsing. I prefer displaying all. In that case, delays could get zero or inf values as they are impossible values. Or sure, display an error message.