[MANAGEMENT. POLICIES]. Absence checks

[AndreyNenashev]

Current policies implementation doesn't consider absence checks. For example:
Create a policy which allows to create terms, manage terms where a user is an owner and manage all terms where no owner assigned.

{
    "statements": [
        {
      "resource": {
        "type": "TERM",
        "conditions": {
          "any": [
            {
              "is": "term:owner"
            },
            {
              "eq": {
                "term:owner": null
              }
            }
          ]
        }
      },
      "permissions": [
        "ALL"
      ]
        },
        {
            "resource": {
                "type": "MANAGEMENT"
            },
            "permissions": [
                "TERM_CREATE"
            ]
        }
    ]
}