QtCore: Use Q_NULLPTR instead of 0 in smart pointer headers

This is in preparation of adding -Wzero-as-null-pointer-constant (or similar)
to the headers check.

Not caught by the headersclean check, because they are in template code.

Task-number: QTBUG-45291
Change-Id: I7294404225a19a1c58f91e6e47a9d650179ea83c
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>