message: Fix assert_return cases when no return value is expected
diff: @@ -83,8 +83,11 @@ public:
  }
  const auto& expected = cmd.at("expected");
  - if (expected.empty() && !result->stack.empty())
  + if (expected.empty())
  {
  + if (result->stack.empty())
  + pass();
  + else
  fail("Unexpected returned value.");
  continue;
  }
  