Always print empty table if nothing is found by dhcpchecker

Printing nothing leads to false conclusions that dhcpchecker
failed to execute command. Printing empty table when there is
no dhcp servers found should be much more user friendly

Change-Id: I99041375fd1a052a796efcba530880a5149733b8
Closes-Bug: 1377174
Merge "Always print empty table if nothing is found by dhcpchecker"