Should Definitions be allowed to have empty contents? Must resolve.

[VisLab]

The tests assume that they should have non-empty contents:

DEFINITION_INVALID definition-invalid-missing-inner-group: Example:

{
  "defs": {
    "HED": {
      "def1": "(Definition/Apple)"
    }
  }
}

There is an argument that this should be allowed to so that you can anchor onsets and offsets without actually having to put content in the definition. The hed-validator doesn't treat this as an error, but the hed-python validator does at this point..