tools: ynl: avoid print_field when there is no reply

tools: ynl: avoid print_field when there is no reply

When request a none support device operation, there will be no reply.
In this case, the len(desc) check will always be true, causing print_field
to enter an infinite loop and crash the program. Example reproducer:

  # ethtool.py -c veth0

To fix this, return immediately if there is no reply.

Fixes: f3d07b02b2b8 ("tools: ynl: ethtool testing tool")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Link: https://patch.msgid.link/20251024125853.102916-1-liuhangbin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>