MAAS2: handle fields that might be missing or null from the API

[babbageclunk]

I asked the MAAS team to look through a list of all the fields that we're using in the MAAS v2 API and tell us which of them were optional or nullable. These changes are based on the information they gave us back.

Based on the spreadsheet here: https://docs.google.com/a/canonical.com/spreadsheets/d/1qysFkvJwz3w19B3kYcnlokwqdTb82HGCrF4pUk6gtfA/edit?usp=sharing

[dimitern]

LGTM

[babbageclunk]

$$merge$$

[jujubot]

Status: merge request accepted. Url: http://juju-ci.vapour.ws:8080/job/github-merge-juju-gomaasapi