FDC ID is not allowed to be empty for new food import

[mikhail5555]

Issue:

Some foods don't have an FDC ID, especially produces (In this example Lime Peel)
fdc_id is not nullable which throws a validation error if you try to add a food without it.

https://github.com/TandoorRecipes/open_data_plugin/blob/master/models.py#L114

In some other models it seems to be nullable:
https://github.com/TandoorRecipes/open_data_plugin/blob/master/models.py#L82

Request:

{
   "version":{
      "id":1,
      "name":"base",
      "code":"base"
   },
   "slug":"food-lime-peel-raw",
   "name":"Lime Peel",
   "plural_name":"Lime Peel",
   "store_category":{
      "id":1,
      "version":{
         "id":1,
         "name":"base",
         "code":"base",
         "comment":""
      },
      "slug":"category-produce",
      "name":"Produce",
      "description":"",
      "comment":"Vegetables and fruits. Could be split into those categories.",
      "created_by":"szeraax"
   },
   "preferred_unit_metric":null,
   "preferred_shopping_unit_metric":null,
   "preferred_unit_imperial":null,
   "preferred_shopping_unit_imperial":null,
   "properties":[
      {
         "property":{
            "id":6,
            "version":{
               "id":1,
               "name":"base",
               "code":"base",
               "comment":""
            },
            "slug":"property-carbohydrates",
            "name":"Carbohydrates",
            "unit":"g",
            "fdc_id":1005,
            "comment":"",
            "created_by":"vabene1111"
         },
         "property_amount":"16"
      },
      {
         "property":{
            "id":4,
            "version":{
               "id":1,
               "name":"base",
               "code":"base",
               "comment":""
            },
            "slug":"property-calories",
            "name":"Calories",
            "unit":"kcal",
            "fdc_id":1008,
            "comment":"",
            "created_by":"vabene1111"
         },
         "property_amount":"47"
      },
      {
         "property":{
            "id":5,
            "version":{
               "id":1,
               "name":"base",
               "code":"base",
               "comment":""
            },
            "slug":"property-proteins",
            "name":"Proteins",
            "unit":"g",
            "fdc_id":1003,
            "comment":"",
            "created_by":"vabene1111"
         },
         "property_amount":"1.5"
      },
      {
         "property":{
            "id":7,
            "version":{
               "id":1,
               "name":"base",
               "code":"base",
               "comment":""
            },
            "slug":"property-fats",
            "name":"Fats",
            "unit":"g",
            "fdc_id":1004,
            "comment":"",
            "created_by":"vabene1111"
         },
         "property_amount":"0.3"
      }
   ],
   "fdc_id":"",
   "comment":"",
   "properties_food_amount":"100",
   "properties_food_unit":{
      "id":1,
      "version":{
         "id":1,
         "name":"base",
         "code":"base",
         "comment":""
      },
      "slug":"unit-g",
      "name":"g",
      "plural_name":"g",
      "base_unit":"G",
      "type":"WEIGHT",
      "comment":"",
      "created_by":"vabene1111"
   },
   "properties_source":"Same as food-lemon-peel-raw"
}

Response:

{"fdc_id":["This field may not be blank."]}

Screenshot:

Proposed solution:

Make the field nullable for foods that don't have one