QMetaType: Document the default ctor

It wasn't showing up in documentation.
Amends 33cd680ddb.

Pick-to: 6.4 6.2
Change-Id: Ifb5d38fd3d4eb2ecd109ce53809fae5382916dff
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/corelib/kernel/qmetatype.cpp

@@ -3108,6 +3108,13 @@ static const QtPrivate::QMetaTypeInterface *interfaceForType(int typeId)
     return iface;
 }
 
+/*!
+     \fn QMetaType::QMetaType()
+     \since 6.0
+
+     Constructs a default, invalid, QMetaType object.
+*/
+
 /*!
      \fn QMetaType::QMetaType(int typeId)
      \since 5.0