DataSourceSmartOS list() should always return a list

If customer_metadata is empty, a stack trace is seen on the console:

2018-04-12 16:01:18,302 - DataSourceSmartOS.py[DEBUG]: Writing "V2 13 d8094091 1055c865 KEYS
" to metadata transport.
2018-04-12 16:01:18,382 - DataSourceSmartOS.py[DEBUG]: Read "V2 16 b23eb5d0 1055c865 SUCCESS" from metadata transport.
2018-04-12 16:01:18,382 - DataSourceSmartOS.py[DEBUG]: No value found.
2018-04-12 16:01:18,382 - handlers.py[DEBUG]: finish: init-local/search-SmartOS: FAIL: no local data found from DataSourceSmartOS
2018-04-12 16:01:18,382 - util.py[WARNING]: Getting data from <class 'cloudinit.sources.DataSourceSmartOS.DataSourceSmartOS'> failed
2018-04-12 16:01:18,382 - util.py[DEBUG]: Getting data from <class 'cloudinit.sources.DataSourceSmartOS.DataSourceSmartOS'> failed
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/cloudinit/sources/__init__.py", line 447, in find_source
    if s.get_data():
  File "/usr/lib/python3/dist-packages/cloudinit/sources/__init__.py", line 121, in get_data
    return_value = self._get_data()
  File "/usr/lib/python3/dist-packages/cloudinit/sources/DataSourceSmartOS.py", line 238, in _get_data
    md[ci_noun] = self.md_client.get(smartos_noun, strip=strip)
  File "/usr/lib/python3/dist-packages/cloudinit/sources/DataSourceSmartOS.py", line 640, in get
    if self.is_b64_encoded(key):
  File "/usr/lib/python3/dist-packages/cloudinit/sources/DataSourceSmartOS.py", line 628, in is_b64_encoded
    self._init_base64_keys(reset=reset)
  File "/usr/lib/python3/dist-packages/cloudinit/sources/DataSourceSmartOS.py", line 594, in _init_base64_keys
    if 'base64_all' in keys:
TypeError: argument of type 'NoneType' is not iterable
2018-04-12 16:01:18,391 - main.py[DEBUG]: No local datasource found

To reproduce:

# vmadm create <<EOF
{
  "alias": "ci",
  "brand": "bhyve",
  "ram": "1024",
  "vcpus": "2",
  "disks": [
    {
      "image_uuid": "4260219e-09d7-c1b3-908c-947839b1d10c",
      "boot": true,
      "model": "virtio"
    }
  ]
}
EOF

As the zone boots, the errors shown above will appear on the console. If using a SmartOS platform image that has a date stamp of 20180412 or later, this information will be visible in /zones/$uuid/logs/console.log. The image uuid above is a custom image I created based on the Ubuntu certified 16.04 image.