fix(interaction-output): adjust storing behavior and return type of `…

…value` method (#1211)

* fix(interaction-output): allow stored value to be `null`

* fix(interaction-output): restrict value method to DataSchemaValues

packages/core/src/interaction-output.ts

@@ -77,9 +77,11 @@ export class InteractionOutput implements WoT.InteractionOutput {
         return data;
     }
 
-    async value<T>(): Promise<T> {
+    async value<T extends WoT.DataSchemaValue>(): Promise<T> {
         // the value has been already read?
-        if (this.parsedValue != null) return this.parsedValue as T;
+        if (this.parsedValue !== undefined) {
+            return this.parsedValue as T;
+        }
 
         if (this.dataUsed) {
             throw new Error("Can't read the stream once it has been already used");