QtConcurrent: Use Q_NULLPTR instead of 0 in all public headers

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

Since QtConcurrent is basically all-templates, not all uses of 0 as nullptr
might have been detected by the headersclean check.

Task-number: QTBUG-45291
Change-Id: Iffcabdbab26b56597700346cd039d0c32c4fddca
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/concurrent/qtconcurrentiteratekernel.h

@@ -127,7 +127,7 @@ public:
     inline ResultReporter(ThreadEngine<void> *) { }
     inline void reserveSpace(int) { }
     inline void reportResults(int) { }
-    inline void * getPointer() { return 0; }
+    inline void * getPointer() { return Q_NULLPTR; }
 };
 
 inline bool selectIteration(std::bidirectional_iterator_tag)

src/concurrent/qtconcurrentthreadengine.h

@@ -126,7 +126,7 @@ class ThreadEngine : public virtual ThreadEngineBase
 public:
     typedef T ResultType;
 
-    virtual T *result() { return 0; }
+    virtual T *result() { return Q_NULLPTR; }
 
     QFutureInterface<T> *futureInterfaceTyped()
     {