isDefined does not support != true and !isDefined

[mellistibco]

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[] feature request
[] support request
[] general question

Current behavior (how does the issue manifest):
When defining a link I cannot negate a boolean value or check against a value. For example:

isDefined($activity[rest_2].result.photoUrlz) != true
!isDefined($activity[rest_2].result.photoUrlz)

Expected behavior:
I should be able to compare to a bool or negate the value.

Minimal steps to reproduce the problem (not required if feature enhancement):
Define a linker:

          {
            "from": "rest_2",
            "to": "log_3",
            "type": "expression",
            "value": "isDefined($activity[rest_2].result.photoUrlz) != true"
          }