System.ConditionExists Component

Summary

I am asking why this built-in component always pass to the exists action.

Content

Here how i use it :

conditionExists:
    component: "System.ConditionExists"
    properties:
      variable: "destinDate"
    transitions:
      actions:
        exists: "done"
        notexists: "askForTripType"

Always it take the action of exists wich is "done"  here. this is my payload where the "destinDate"  variable is null but the component act like it's not. 

"destinAir":"MIR",
"user.botsUserError":false,
"profile.locale":"fr-FR",
"destinDate":null,
"profile.timezoneOffset":-7200000,
"tripType":null,
"profile.firstName":"Ameur",
"departAir":"CDG",
"departDate":{  
   "date":1534582854109,
   "entityName":"DATE",
   "originalString":"08/18/2018"
},
"system.processedUserMessage":true
},
"currentDialogState":"done"

Any idea please