SensorHistories Data can be null, will cause crash on SerialiseData()

[VladimirAkopyan]

Turns out that this isn't the end! A SensorHIstory with a data collection of null will actually crash on Merge() attempt. This is yet another error that happens during sync and needs to be corrected!

[VladimirAkopyan]

What I did was a work-around. SensorHistory.Data is produced either when we:

• Get data from local database and then de-serialise it - this produces a collection with no entries, if there aren't any
• Query the server and it sends an empty collection - this will actually produce null. The application just needs to check data received form server for that.

[VladimirAkopyan]

Found that the error isn't just the app's fault - server has replied with: https://greenhouseapi.azurewebsites.net/api/SensorsHistory/CD87474B-3C6A-4C87-938F-BC586B5540C9/1507248000/15

[
{
        "Data": null,
        "LocationID": "facb7a7b-f8c0-4514-acc8-665304391b61",
        "SensorID": "b4ec891c-2b69-464b-9cae-8ed95d6bb244",
        "TimeStamp": "2017-10-08T00:00:00+02:00",
        "UploadedAt": "2017-10-07T04:13:43.0682831+00:00"
    },
    {
        "Data": null,
        "LocationID": "facb7a7b-f8c0-4514-acc8-665304391b61",
        "SensorID": "7696b336-737a-4521-91c9-84f16e6ea930",
        "TimeStamp": "2017-10-08T00:00:00+02:00",
        "UploadedAt": "2017-10-07T04:13:43.0682831+00:00"
    },
    {
        "Data": null,
        "LocationID": "facb7a7b-f8c0-4514-acc8-665304391b61",
        "SensorID": "afd4b2c7-0e1d-48cf-896a-1b528d21e5c4",
        "TimeStamp": "2017-10-08T00:00:00+02:00",
        "UploadedAt": "2017-10-07T04:13:43.0682831+00:00"
    }
]

Meanwhile in the database these items have data!

[VladimirAkopyan]

• When we get null SensorData, this needs to be logged as an error, but the app should keep going. Just replace null with a valid collection.
• The SensorHistory download method must be broken down.