Fails in get_networks if filter unset

Considering the following:

    context = context.get_admin_context()
    plugin = manager.QuantumManager.get_plugin()
    plugin.get_networks(context):

fails with:

Traceback (most recent call last):
  File "/tmp/quantum-usage-audit", line 64, in <module>
    for network in plugin.get_networks(context):
  File "/opt/stack/quantum/quantum/plugins/openvswitch/ovs_quantum_plugin.py", line 477, in get_networks
    nets = self._filter_nets_l3(context, nets, filters)
  File "/opt/stack/quantum/quantum/db/l3_db.py", line 712, in _filter_nets_l3
    vals = filters.get('router:external', [])
AttributeError: 'NoneType' object has no attribute 'get'

This is because _filter_nets_l3 does not check filters to be None.