samples: net: cellular_modem: fix division by zero in sample_echo_packet

A hard fault occurs when the `sample_echo_packet` function attempts
to print the average time per successful echo and no packets have
been sent, resulting in a division by zero error. This fix adds a check
to ensure that `packets_sent` is greater than 0 before performing
the division. This prevents the system from halting due to a usage fault.

Signed-off-by: Natalia Pluta <pluta.natalia.m@gmail.com>