QString/QByteArray::isDetached() returns false for null strings

Currently QString/QByteArray::isDetached() returns false for null strings, and in some cases for empty strings, IIUC that's because it's implemented as:

bool QString::isDetached() { return !d.isShared(); } // d is DataPointer
bool QArrayDataPointer::isShared() { return !d || d->isShared(); }

Logically a null string is detached/not-shared because it has no data array to begin with.

Changing QDP::isShared() to return false when there is no d pointer

 return d && d->isShared() 

could solve the issue.

(And while at it, we could add a new method:

QString/QByteArray::isShared() { return d.isShared(); }

, the double negative "string is detached when its DataPointer is not shared " is harder to reason about than "string is shared when its DataPointer is shared").