createEmpty : true in aggregateWindow does not work for 'selector' functions

Steps to reproduce:

When performing queries with missing data, for example

from(bucket: "bucket")
  |> range(start: v.timeRangeStart, stop: v.timeRangeStop)
  |> filter(fn: (r) => r["_measurement"] == "measurement")
  |> filter(fn: (r) => r["_field"] == "field")
  |> aggregateWindow(every: v.windowPeriod, fn: max, createEmpty: true)

if fn parameter in aggregateWindow is part of aggregates things work as expected, however when using selectors, such as last, first, min, max, createEmpty: true does not have any effect

Expected behavior:
Returned datasets would include null in _value column for missing timestamps

Actual behavior:
Returned datasets do NOT include null in _value column for missing timestamps

Environment info:

• System info: Linux 4.15.0-135-generic x86_64
• InfluxDB version: 2.0.7

Start by running the provided Flux query with aggregateWindow and compare aggregate functions with selector functions such as max, min, first, and last when createEmpty is true. Confirm the behavior for missing timestamps and inspect the aggregateWindow implementation and its tests. Done means selector functions produce rows with null in _value for empty windows.