Make null/undefined values deserialize correctly, especially for optional fields

src/serialization/deserialize.js

@@ -10,6 +10,8 @@ function isReference(serializedData) {
 	return (serializedData != null && serializedData._d_sT === "cTR");
 }
 
+/* FIXME: Pass both key and value to custom deserializer */
+
 module.exports = function createDeserializer(topLevelType, topLevelData, typeMap, options = {}) {
 	let seen = new Map();
 	let placeholders = createPlaceholderManager();
@@ -61,7 +63,9 @@ module.exports = function createDeserializer(topLevelType, topLevelData, typeMap
 	}
 
 	function deserializeEntry(rule, serializedData) {
-		if (rule._isCustomType) {
+		if (serializedData == null) {
+			return customDeserializer(serializedData);
+		} else if (rule._isCustomType) {
 			return deserializeInstanceOrReference(rule, serializedData);
 		} else if (rule._isCustomRegistryType) {
 			return deserializeInstanceOrReference(rule._registry._getType(rule._name), serializedData);