(fix) Fixes serialization in the presence of undefined

JSON does not support `undefined` as a value, but we do need to put
some value there as otherwise we can't properly reify the structure
(we iterate over keys and copy those descriptors to the reified structure).
For this reason, undefined is now serialised as `null`.

docs/source/en/data/result/to-json.md

@@ -15,5 +15,8 @@ See the docs for `core/adt/derivations/serialization` for more details.
     Result.Ok(1).toJSON();
     // ==> { '@@type': 'folktale:Data.Result', '@@tag': 'Ok', '@@value': { value: 1 } }
 
-    Result.Error().toJSON();
-    // ==> { '@@type': 'folktale:Data.Result', '@@tag': 'Error', '@@value': { value: 1 } }
+    Result.Error(1).toJSON();
+    // ==> { '@@type': 'folktale:Data.Result', '@@tag': 'Error', '@@value': { value: 1 } }
+
+    Result.Error(undefined).toJSON();
+    // ==> { '@@type': 'folktale:Data.Result', '@@tag': 'Error', '@@value': { value: null } }

src/core/adt/derivations/serialization.js

@@ -88,8 +88,9 @@ const parseValue = (parsers) => (value) => {
  * type: ('a) => JSON
  */
 const serializeValue = (value) =>
-  value !== null && typeof value.toJSON === 'function' ? value.toJSON()
-  : /* otherwise */                                      value;
+  value === undefined                                  ?  null
+: value !== null && typeof value.toJSON === 'function' ?  value.toJSON()
+: /* otherwise */                                         value;
 
 
 // --[ Implementation ]-------------------------------------------------