tc/taprio: don't print netlink attributes which weren't reported by the kernel

tc/taprio: don't print netlink attributes which weren't reported by the kernel

When an admin schedule is pending and hasn't yet become operational, the
kernel will report only the parameters of the admin schedule in a nested
TCA_TAPRIO_ATTR_ADMIN_SCHED attribute.

However, we default to printing zeroes even for the parameters of the
operational base time, when that doesn't exist.

Fixes: 0dd16449356f ("tc: Add support for configuring the taprio scheduler")
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Acked-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>