Make 'if "something" not in [nonexistent] { ... }' succeed

My argument is checking for the absence of a value in a field that doesn't
exist should return true, currently it is false.

This fixes the case where an event which has never been tagged now works with
this logic:

    grok {
      ...
    }
    if "_grokparsefailure" not in [tags] {
      ...
    }
Merge pull request #682 from bodgit/notin

Make 'if "something" not in [nonexistent] { ... }' succeed