Cusum() function in Timelion returns no values when applied on a log() seriesList with first bucket value null

[GeorgeRD]

Kibana version: 6.8.10

Elasticsearch version: 6.8.10

Server OS version: CentOS 7

Browser version: Chrome 87.0.4280.88 (Official Build) (64-bit)

Browser OS version: Windows 10 Home

Original install method (e.g. download page, yum, from source, etc.): yum

Describe the bug:
The cusum() function in Timelion does not return any results when applied in a logarithmic series with null value in its first point.

Steps to reproduce:

1. Create a derivative seriesList in Timelion (first point in derivative series is null)
2. Apply the log() function in the seriesList and check that results are returned (first point in derivative series is also null)
3. Apply the cusum() function in the logarithmic series. All points returned are null

Workaround:
After the derivative series, apply a condition(operator=eq, if=null, then=1) and then proceed with log() and cusum().

Expected behavior:
Data expected to return for those values present

Screenshots (if relevant):

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context:

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[flash1293]

The issue here isn't the null value in the first bucket, it's because .derivative() is returning negative values which become NaN when log is applied to them (because the logarithm of a negative number isn't a real number). On summing up all of the NaNs, the cumulative sum of them is NaN as well.