firewall: don't return error in DEL if prevResult is not found.
firewall: don't return error in DEL if prevResult is not found.

The CNI spec states that for DEL implementations, "when CNI_NETNS and/or
prevResult are not provided, the plugin should clean up as many resources as
possible (e.g. releasing IPAM allocations) and return a successful response".
This change results in the firewall plugin conforming to the spec by not
returning an error whenever the del method is not provided a prevResult.

Signed-off-by: Erik Sipsma <14369ebac1a3f01314fb9fa84b913874d1ad9450@amazon.com>

containernetworking/plugins,containernetworking/plugins