allowing NULL in elu-core schema fields

metadata_tools/resources/elu-core.json

@@ -19,16 +19,16 @@
       "type": "string"
     },
     "description": {
-      "type": "string"
+      "type":  ["string", "null"]
     },
     "elu_accession": {
-      "type": "string"
+      "type": ["string", "null"]
     },
     "elu_uuid": {
-      "type": "string"
+      "type": ["string", "null"]
     },
     "other_external_id": {
-      "type": "string"
+      "type": ["string", "null"]
     },
     "contacts": {
       "type": "array",
@@ -62,7 +62,7 @@
             "minItems": 1,
             "uniqueItems": true,
             "items": {
-              "type": "string"
+              "type": ["string", "null"]
             }
           }
         },