fix: return MalformedXML for a DeleteObjects request without objects

A DeleteObjects request whose Delete element names no objects was
answered with 200 and an empty DeleteResult. S3 rejects it with
MalformedXML, as the schema requires at least one Object.

Reject the empty list right after the body is parsed, next to the
1000 key limit check. Update the unit and integration tests that
expected the empty result and enable the REST test that covers it.

Fixes #2109

tests/test_rest_delete_objects.sh

@@ -36,9 +36,6 @@ source ./tests/setup.sh
 
 # tags: curl, DeleteObjects, malformed-message
 @test "REST - DeleteObjects - no objects added to payload" {
-  if [ "$DIRECT" != "true" ]; then
-    skip "https://github.com/versity/versitygw/issues/2109"
-  fi
   run setup_bucket_and_file_v3 "$BUCKET_ONE_NAME"
   assert_success
   read -r bucket_name file_name <<< "$output"