Need ability to check for null field

I am looking to construct a query which alerts when a field is null. Currently it seems the only way to do this is to use a `default()` node to set the value to something unlikely to be present, and then check for that specific value. But this is a poor hack as if the field can have any value, then it's possible to have a real value end up the same as your null-default value.

The issue names no files, tests, or entry points. Start by tracing how Kapacitor query fields and alert conditions represent missing or null values, then determine the intended null semantics and user-facing query syntax. Done means a field can be checked for null without relying on a sentinel default value.

Written by the indexing model from the issue text.