message: Fix swagger quality issues with access review APIs | Mark fields as nullable
diff: "type": "string",
  "format": "date-time",
  "description": "The DateTime when the review is scheduled to be start. This could be a date in the future. Required on create.",
  - "x-nullable": false
  + "x-nullable": true
  },
  "endDate": {
  "type": "string",
  "format": "date-time",
  "description": "The DateTime when the review is scheduled to end. Required if type is endDate",
  - "x-nullable": false
  + "x-nullable": true
  }
  },
  "description": "Recurrence Range of an Access Review Schedule Definition."
  "readOnly": true,
  "format": "date-time",
  "description": "Date Time when a decision was taken.",
  - "x-nullable": false
  + "x-nullable": true
  },
  "reviewedBy": {
  "readOnly": true,
  "x-ms-client-flatten": true,
  "$ref": "#/definitions/AccessReviewActorIdentity",
  - "description": "Details of the approver."
  + "description": "Details of the approver.",
  + "x-nullable": true
  },
  "applyResult": {
  "type": "string",
  "readOnly": true,
  "format": "date-time",
  "description": "The date and time when the review decision was applied.",
  - "x-nullable": false
  + "x-nullable": true
  },
  "appliedBy": {
  "readOnly": true,
  "x-ms-client-flatten": true,
  "$ref": "#/definitions/AccessReviewActorIdentity",
  - "description": "Details of the approver."
  + "description": "Details of the applier.",
  + "x-nullable": true
  }
  },
  "description": "Approval Step."
  