CompareAgainstLiteralZero: fix SFINAE considerations

This issue arose during the comparison of two different ordering types.
When comparing QPartialOrdering::Less to QStrongOrdering::Less, an
unintended overload was considered due to the SFINAE ctor-overload of
CompareAgainstLiteralZero. For example:

static_assert(QPartialOrdering::Less == QStrongOrdering::Less);

would consider:

friend constexpr bool operator==(QtPrivate::CompareAgainstLiteralZero,
                                 QStrongOrdering rhs) noexcept

as an overload. To address this, a stricter approach is now used
by triggering the SFINAE-check on std::nullptr_t instead.
This resolves the ambiguity while still rejecting std::nullptr_t
as intended. As the compiler is unable to resolve this automatically,
this refactoring is required.

Pick-to: 6.5 6.2
Change-Id: I9ab7e55d2822980198f38f5a66143387999a4d94
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/corelib/global/qcompare_impl.h

@@ -22,7 +22,7 @@ public:
     using SafeZero = void (CompareAgainstLiteralZero::*)();
     Q_IMPLICIT constexpr CompareAgainstLiteralZero(SafeZero) noexcept {}
 
-    template <typename T, std::enable_if_t<!std::is_same_v<T, int>, bool> = false>
+    template <typename T, std::enable_if_t<std::is_null_pointer_v<T>, bool> = true>
     CompareAgainstLiteralZero(T) = delete;
 };