fix: Let Representations always have a body

This is relevant when the request has a content-type
but no data.

test/unit/storage/DataAccessorBasedStore.test.ts

@@ -164,6 +164,7 @@ describe('A DataAccessorBasedStore', (): void => {
       metadata: new RepresentationMetadata(
         { [CONTENT_TYPE]: 'text/plain', [RDF.type]: DataFactory.namedNode(LDP.Resource) },
       ),
+      isEmpty: false,
     };
   });