fix(InteractionOutput): adjust implementation to spec

lib/src/core/implementation/interaction_output.dart

@@ -8,6 +8,7 @@ import "dart:typed_data";
 
 import "../definitions/data_schema.dart";
 import "../definitions/form.dart";
+import "../exceptions.dart";
 import "../scripting_api.dart" as scripting_api;
 import "content.dart";
 import "content_serdes.dart";
@@ -38,6 +39,10 @@ class InteractionOutput implements scripting_api.InteractionOutput {
 
   @override
   Future<ByteBuffer> arrayBuffer() async {
+    if (dataUsed) {
+      throw NotReadableException("Data has already been read");
+    }
+
     _dataUsed = true;
     return _content.byteBuffer;
   }
@@ -52,8 +57,11 @@ class InteractionOutput implements scripting_api.InteractionOutput {
       return existingValue.value;
     }
 
-    // TODO(JKRhb): Should a NotReadableError be thrown if schema is null?
-    //              C.f. https://w3c.github.io/wot-scripting-api/#the-value-function
+    if (schema == null) {
+      throw NotReadableException(
+        "Can't convert data to a value because no DataSchema is present.",
+      );
+    }
 
     final value = await _contentSerdes.contentToValue(
       _content,

test/core/interaction_output_test.dart

@@ -24,7 +24,12 @@ void main() {
         ]),
       );
 
-      final interactionOutput = InteractionOutput(content, contentSerdes);
+      final interactionOutput = InteractionOutput(
+        content,
+        contentSerdes,
+        null,
+        const DataSchema(),
+      );
 
       final value1 = await interactionOutput.value();
       expect(value1, inputValue);
@@ -46,7 +51,12 @@ void main() {
         ]),
       );
 
-      final interactionOutput = InteractionOutput(content, contentSerdes);
+      final interactionOutput = InteractionOutput(
+        content,
+        contentSerdes,
+        null,
+        const DataSchema(),
+      );
 
       final value1 = await interactionOutput.value();
       expect(value1, inputValue);