Canvas needs to display results not available during that time period instead of math error

[bhavyarm]

Kibana version: 6.7.0 BC4

Browser version: chrome latest

Browser OS version: OS X

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

Describe the bug: If user adds a timefilter to sample flight data workpads, some of the elements display ** expression failed function math should return number** instead of telling the user there are no results in the 24 hour time period which the timefilter adds as default time range.

Either that or tell the user to change the time to be able to see results

Steps to reproduce:

1. Add sample data and open sample data flights workpad
2. Click to add elements - notice that some of the elements display expression failed math error

[elasticmachine]

Pinging @elastic/kibana-canvas

[timductive]

The underlying issue here is how null values are interpreted, closing this issue in favor of: #23087