QVariant: Fix MSVC compiler warning about unused variable null.

Use static method isnull().
Introduced by ca26fefc06.

Change-Id: I752f8d8a5ec473dcc5f50fbda47d061b8d3a7e13
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>

src/corelib/kernel/qvariant_p.h

@@ -282,8 +282,7 @@ public:
     template<typename T>
     bool delegate(const T*)
     {
-        CallIsNull<T> null;
-        return null.isNull(m_d);
+        return CallIsNull<T>::isNull(m_d);
     }
     // we need that as sizof(void) is undefined and it is needed in HasIsNullMethod
     bool delegate(const void *) { return m_d->is_null; }