util: Add Expected<void, E> specialization

This is not needed, but a bit closer to the std lib, because
std::monostate is no longer leaked through ValueType from the value()
method.

src/test/util_expected_tests.cpp

@@ -66,6 +66,8 @@ BOOST_AUTO_TEST_CASE(expected_error)
 {
     Expected<void, std::string> e{};
     BOOST_CHECK(e.has_value());
+    [&]() -> void { return e.value(); }(); // check value returns void and does not throw
+    [&]() -> void { return *e; }();
 
     e = Unexpected{"fail"};
     BOOST_CHECK(!e.has_value());