When the podSelector of a Network Policy Spec is updated from '{}'
to any other value, the pods that are not supposed to be enforced
by the policy anymore do not have their Security Group updated
to the default one. This commit fixes the issue by also taking into
account the possible value of '{}' in the podSelector NP spec.
Change-Id: I35519acfdf8ef250880e36bcf789c063ba86b31e
Closes-Bug: 1826548