Fix missing None judgement in rollback
Fix missing None judgement in rollback

in vnflcm_drive._term_resource_update(),
it is checking if 'affected_vnfcs' is existing in
vnf_lcm_op_occs.resource_changes.

However, 'affected_vnfcs' may be None,
and cause an None type error in next line,
using len() for affect_vnfcs.
(same to other 2 variables below)

This patch is adding None judgement before
len()

Closes-Bug: #1922191
Change-Id: I19aedf0940d8b8d51fa5bd1d5c5092655d7970b0

openstack/tacker,openstack/tacker,stackforge/tacker,stackforge/tacker,openstack/tacker