Kibana version: 7.12

Describe the bug:

When using 3 out of the 4 new calculation functions (counter rate, differences, and moving average) in a datatable, the value sorting seems to not properly work:

Cumulative sum works ok as it does not have null values.

Steps to reproduce:

1. Use the logs sample data
2. Open the Lens editor and pick the Datatable visualization
3. Add @timestamp to the Break down by section
4. Configure a Moving Average of Percentile as Metric
5. Sort the Moving Average column (either ascending or descending)
6. -- values float in the table without being pushed down.

Expected behavior:

Sorting should work as other operations where -- values are pushed down at the bottom (or considered as minimum value, either way is ok) and the remaining numeric values gets sorted accordingly.