diagnostics: Fix panic when a NIC doesn't have a mac address (#7889)
diagnostics: Fix panic when a NIC doesn't have a mac address (#7889)

Signed-off-by: Kaige Ye <2c2cc9b893cb6323e0f662f41aa4cf4b1e13d787@gmail.com>
BusyJay/tikv,BusyJay/tikv,pingcap/tikv,c4pt0r/tikv,BusyJay/tikv,BusyJay/tikv,pingcap/tikv