Window node does not populate data prior to task definition

The tick script expects window of the last 4h values to be emitted every 1m for further processing.
However just after task definition the emitted data is empty, and then it gets populated with more values, starting from 1m range.

Expected behavior is that 4h window is emitted right from the start of the task.

```
stream
|from()
.measurement('cpu')
|window()
.period(4h)
.every(1m)
|httpOut('4h')
```

```
kapacitor stats general
ClusterID: 14aecaee-b88b-420b-9eaa-4e572a0035f6
ServerID: 30a9d71d-e00a-4833-b3bb-e6dd88016b93
Host: localhost
Tasks: 1
Enabled Tasks: 1
Subscriptions: 6
Platform: OSS
Version: v1.5.0
```

```
influx -version
InfluxDB shell version: v1.5.3
```

Start by reproducing the provided tick script with the 4h window and 1m emission settings against the reported Kapacitor and InfluxDB versions. Inspect the window node's behavior immediately after task definition; done means the first emitted window contains the prior 4h of values rather than starting empty and growing from 1m.

Written by the indexing model from the issue text.