don't require datetime to be null (#21)

Author: keewis

json-schema/schema.json

@@ -31,11 +31,6 @@
                 },
                 {
                   "$ref": "#/definitions/fields"
-                },
-                {
-                  "properties": {
-                    "datetime": {"const": null}
-                  }
                 }
               ]
             }