Empty values should not be empty strings

In various instances in the API I see missing values being encoded as empty strings.

One example, is in GET /taskWorkItems, for provisioning workitems:

      "project": [
        ""
      ],
      "branch": "",

This is confusing because empty string is not a valid project or branch name.

Instead when a simple value is missing, it should be encoded as the special null JSON value. When a list is empty, it should just be encoded as an empty array. The previous would become:

      "project": [
      ],
      "branch": null,

P.S. For the specific example, project shouldn't even be a list in the first place, but that is reported separately in COIN-942.