There are at least two cases where napi_id may not present and the
napi_id should be elided:
1. Queues could be created, but napi_enable may not have been called
yet. In this case, there may be a NAPI but it may not have an ID and
output of a napi_id should be elided.
2. TX-only NAPIs currently do not have NAPI IDs. If a TX queue happens
to be linked with a TX-only NAPI, elide the NAPI ID from the netlink
output as a NAPI ID of 0 is not useful for users.
Signed-off-by: Joe Damato <jdamato@xxxxxxxxxx>