Comparing with previous data if it none

Hi!
I am trying to create a relative alert comparing with last data.
I wrote the script and it has worked fine still I needed comparing new data with none.
I have following script:
```
var current = batch
|query('select mean(total) as total_new from "Debug"."autogen"."debug" FILL(0)')
.period(1m)
.every(30s)
.align()
.groupBy('Message')
|log()

var past = batch
|query('select mean(total) as total_old from "Debug"."autogen"."debug" FILL(0)')
.period(1m)
.every(30s)
.align()
.offset(1m)
.groupBy('Message')
|shift(1m)
|log()

var trigger = past
|join(current)
.as('past', 'current')
|eval(lambda: float("current.total_new" - "past.total_old"))
.keep()
.as('value')
|alert()
.category('relative')
.crit(lambda: "value" > 0)
.message(message)
.stateChangesOnly()
.log('/var/log/alerts/batch_relative.log')
|log()
```
So the script is comparing current data with the data in previous 1 min interval then if value is greater than previous generates an alert.

**However, this script does not work if there is no data in previous interval, because Influx returns none while I need an alert in this case too (when in previous interval no data, but in current is).**

Is there any workaround for my case?

The issue names no repository files or tests; begin by reproducing the supplied Kapacitor script with an empty previous interval and inspect the query, shift, and join behavior. Done means establishing whether a current point can produce an alert when the previous interval has no data, with the resulting behavior or workaround documented.

Written by the indexing model from the issue text.