/// Finds the key \p val and return the item found
/** \anchor cds_intrusive_SplitListSet_rcu_get
The function searches the item with key equal to \p val and returns the pointer to item found.
- If \p val is not found it returns \p NULL.
+ If \p val is not found it returns \p nullptr.
Note the compare functor should accept a parameter of type \p Q that can be not the same as \p value_type.