wifi: wifi-he-network example checks if the throughput is greater than zero before failing

Currently, the wifi-he-network example fails when you run for instance

./waf --run "wifi-he-network --distance=20 --frequency=2.4 --mcs=11 --udp=false"

The example fails because the first obtained throughput is zero and in the following execution, with a shorter GI, the example expects a higher throughput while it is again zero.

Then, the wifi-he-network example should check if the obtained throughout is greater than zero before failing due to unexpected value.