virttest.env_process: Fix AttributeError when nics parameter is undefined

Some cases don't use nics parameter, but the preprocess
function in env_process module would read nics config and
init network in every case.

This patch is a workaround when nics is undefined. It makes
autotest ignore network initiation process in bridge env.
Signed-off-by: NQingtang Zhou <qzhou@redhat.com>