Backend delta for one-point datasets fails

[n-wbrown]

Describe the bug The backend "delta" operator fails when a data set with only a single value is received.

To Reproduce Steps to reproduce the behavior:

1. Create a query for a value that will return only a single data point. An enum that doesn't change is a good candidate for this.
2. Add the delta operator to this data.
3. Create an alert on an interval that will only return a single datapoint.
4. Use "Test rule" to see it fail.

Expected behavior A delta of a single point should simply return 0.

Additional context This is a backend issue.