QMetaType: Document the default ctor

It wasn't showing up in documentation.
Amends 33cd680ddbaccf6139e215d851a39e657ae36394.

Change-Id: Ifb5d38fd3d4eb2ecd109ce53809fae5382916dff
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
(cherry picked from commit 47576af3489690551915b3ee3d15affd5f8fb579)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/kernel/qmetatype.cpp

@@ -2984,6 +2984,13 @@ 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