If parent_table None call exception (napalm-automation#939)

Hi, i have this situation when ipv6 is turned off:
$ napalm --user oxi --password gdfw23 --vendor nxos  10.143.36.8 call get_interfaces_ip
2019-03-07 15:08:10,226 - napalm - ERROR - method - Failed: 'NoneType' object has no attribute 'get'

================= Traceback =================

Traceback (most recent call last):
  File "/opt/py3.6/bin/napalm", line 10, in <module>
    sys.exit(main())
  File "/opt/py3.6/lib/python3.6/site-packages/napalm/base/clitools/cl_napalm.py", line 309, in main
    run_tests(args)
  File "/opt/py3.6/lib/python3.6/site-packages/napalm/base/clitools/cl_napalm.py", line 292, in run_tests
    call_getter(device, args.method, **method_kwargs)
  File "/opt/py3.6/lib/python3.6/site-packages/napalm/base/clitools/cl_napalm.py", line 31, in wrapper
    r = func(*args, **kwargs)
  File "/opt/py3.6/lib/python3.6/site-packages/napalm/base/clitools/cl_napalm.py", line 256, in call_getter
    r = func(**kwargs)
  File "/opt/py3.6/lib/python3.6/site-packages/napalm/nxos/nxos.py", line 1091, in get_interfaces_ip
    ipv6_command, "TABLE_intf", "ROW_intf"
  File "/opt/py3.6/lib/python3.6/site-packages/napalm/nxos/nxos.py", line 721, in _get_command_table
    return self._get_reply_table(json_output, table_name, row_name)
  File "/opt/py3.6/lib/python3.6/site-packages/napalm/nxos/nxos.py", line 717, in _get_reply_table
    return self._get_table_rows(result, table_name, row_name)
  File "/opt/py3.6/lib/python3.6/site-packages/napalm/nxos/nxos.py", line 706, in _get_table_rows
    _table = parent_table.get(table_name)
AttributeError: 'NoneType' object has no attribute 'get'

napalm/nxos/nxos.py

@@ -703,6 +703,8 @@ def _get_table_rows(parent_table, table_name, row_name):
         vs
         {'TABLE_vrf': {'ROW_vrf': {'TABLE_adj': {'ROW_adj': {
         """
+        if parent_table is None:
+            return []
         _table = parent_table.get(table_name)
         _table_rows = []
         if isinstance(_table, list):