QVarLengthArray: widen append(p, n)'s contract

It's a bit weird that the (counted-)range-append() allows n < 0, but
requires p != nullptr even when n == 0.

Fix by allowing p == nullptr iff n == 0.

[ChangeLog][QtCore][QVarLengthArray] The counted-range-append()
function (append(ptr, n)) now accepts ptr == nullptr, provided n == 0,
too (was: triggered assertion).

Pick-to: 6.2
Change-Id: Ifb97170a930e97cb8f4194282cada0faf820cc53
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/corelib/tools/qvarlengtharray.h

@@ -481,7 +481,7 @@ Q_INLINE_TEMPLATE bool QVarLengthArray<T, Prealloc>::contains(const AT &t) const
 template <class T, qsizetype Prealloc>
 Q_OUTOFLINE_TEMPLATE void QVarLengthArray<T, Prealloc>::append(const T *abuf, qsizetype increment)
 {
-    Q_ASSERT(abuf);
+    Q_ASSERT(abuf || increment == 0);
     if (increment <= 0)
         return;