fix: Let Representations always have a body

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

test/unit/storage/patch/SparqlUpdatePatcher.test.ts

@@ -19,6 +19,7 @@ function getPatch(query: string): SparqlUpdatePatch {
     data: guardedStreamFrom(prefixedQuery),
     metadata: new RepresentationMetadata(),
     binary: true,
+    isEmpty: false,
   };
 }