fix: return null for zero bytes payloads

Author: JKRhb

lib/src/core/content_serdes.dart

@@ -230,6 +230,11 @@ class ContentSerdes {
 
     final bytes = await content.byteBuffer;
 
+    // TODO: Should null be returned in this case?
+    if (bytes.lengthInBytes == 0) {
+      return null;
+    }
+
     final codec = _getCodecFromMediaType(mimeType);
     if (codec != null) {
       final value = codec.bytesToValue(bytes, dataSchema, parameters);

test/core/content_serdes_test.dart

@@ -45,5 +45,11 @@ void main() {
       () => contentSerdes.valueToContent(42, failingSchema, 'application/json'),
       throwsA(const TypeMatcher<ContentSerdesException>()),
     );
+
+    final testContent3 = _getTestContent('');
+    expect(
+      await contentSerdes.contentToValue(testContent3, null),
+      null,
+    );
   });
 }