app/testpmd: set default RSS key as null

app/testpmd: set default RSS key as null

When creating an RSS rule without specifying a key (see [1]) it is
expected that the device will use the default key.
A NULL key is used to indicate to a PMD it should use
its default key, however testpmd assigns a non-NULL dummy key
(see [2]) instead.
This does not enable testing any PMD behavior when the RSS key is not
specified. This commit fixes this limitation by setting key to NULL.

[1]
RSS rule example without specifying a key:
flow create 0 ingress <pattern> / end actions rss queues 0 1 end / end
[2]
Testpmd default key assignment:
.key= "testpmd's default RSS hash key, "
"override it for better balancing"

Signed-off-by: Ophir Munk <ophirmu@mellanox.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>