Fix the format of `properties` in the MDL

[goldmedal]

Description

The JSON serialized from the MDL now looks like

        {
          "name": "city",
          "type": "varchar",
          "relationship": null,
          "isCalculated": false,
          "noNull": false,
          "expression": null,
          "properties": []
        },

The format of properties is wrong. It should be an object.

        {
          "name": "city",
          "type": "varchar",
          "relationship": null,
          "isCalculated": false,
          "noNull": false,
          "expression": null,
          "properties":  {
               "key": "value"
                ...
           }
        },