Autocomplete nil and empty string

[mdisibio]

Is your feature request related to a problem? Please describe.

Describe the solution you'd like
I'd like tag autocomplete to support expressions like != nil and != "". This is helpful for checking if the span has an attribute but we don't care about the value. I think the way autocomplete works, it might be straighforward to extend the regexes to support these.

Describe alternatives you've considered

Additional context