app/testpmd: enable empty polls in burst stats

app/testpmd: enable empty polls in burst stats

The number of empty polls provides information about available
CPU head room in the presence of continuous polling.

Signed-off-by: Honnappa Nagarahalli <honnappa.nagarahalli@arm.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>
Tested-by: Phil Yang <phil.yang@arm.com>
Tested-by: Ali Alnubani <alialnu@mellanox.com>