handle empty network info in instance cache

The nw_info value [] was being incorrectly equated with None. This caused
infinite recursion when api._get_instance_nw_info returned [] and was wrapped
with @refresh_cache (e.g., _get_instance_nw_info in
nova.network.neutronv2.api.API).

Fixes bug #1251792.

Change-Id: I3034fbb20e790f2d6f22c50b74a9f9dcdf9081aa
(cherry picked from commit 8bc974be50bea6ff6beacda890d9a33fa90059b3)