QFuture: avoid warnings about unreachable code with /W4 in MSVC

... when instantiating the MetaTypeQFutureHelper for QFuture<void>.

Simply enabling /W4 for tst_qfuture also generates this warning, but
also much more. So I'm not providing any unit-test here.

Amends 90d9a86c2e762c9426d0facbd54ed5dbff574ecd.

Fixes: QTBUG-133406
Pick-to: 6.9.0 6.8 6.5
Change-Id: I22a53b872818763b80f333f597ddd19a1c1e30ac
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit ff6bf61b17ee4fc8ca11e016242d7c3399a02045)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/thread/qfuture.h

@@ -437,12 +437,13 @@ template<typename T>
 struct MetaTypeQFutureHelper<QFuture<T>>
 {
     static bool registerConverter() {
-        if constexpr (std::is_same_v<T, void>)
+        if constexpr (std::is_same_v<T, void>) {
             return false;
-
+        } else {
             return QMetaType::registerConverter<QFuture<T>, QFuture<void>>(
                     [](const QFuture<T> &future) { return QFuture<void>(future); });
         }
+    }
 };
 
 }  // namespace QtPrivate