Sensor readings dashboard doesn't show when the database is empty

[jhodapp]

If you stand up a new instance of Ambi, or you truncate the DB tables of all existing data, you'll get an error:

function nil.temperature/0 is undefined

Ambi should handle this error and still display the sensor readings dashboard even when the DB tables are empty.

More details about the error from Phoenix:

[jhodapp]

It's looking like an issue in lib/ambi.ex in the get_last_row() function. When there aren't any rows, Repo.one() returns nil:

defp get_last_row() do
  Repo.one(from s in Ambi.Reading, order_by: [desc: s.id], limit: 1)
end

We need a guard to detect nil and return an empty reading if there aren't any values in the DB.

Another idea is to use Repo.one!() instead and handle the Ecto.NoResultsError runtime error instead. See the Hexdocs for more information.