QCborStreamReader: don't assume that clear() results in isNull()

That QString and QByteArray::clear() shed all capacity() is a
historical artifact we will fix at some point. Don't add more code
that assumes a clear()ed string isNull().

Amends 8af346c1f6.
Amends 1d9137e13f.

Task-number: QTBUG-31283
Task-number: QTBUG-60745
Pick-to: 6.7
Change-Id: Ib0d929325088d3e8e119fee3eafa964a783dc8e9
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Juha Vuolle <juha.vuolle@qt.io>

src/corelib/serialization/qcborstreamreader.h

@@ -150,21 +150,21 @@ public:
     {
         QString dst;
         if (!appendToString(dst))
-            dst.clear();
+            dst = QString{};
         return dst;
     }
     QByteArray toUtf8String()
     {
         QByteArray dst;
         if (!appendToUtf8String(dst))
-            dst.clear();
+            dst = QByteArray{};
         return dst;
     }
     QByteArray toByteArray()
     {
         QByteArray dst;
         if (!appendToByteArray(dst))
-            dst.clear();
+            dst = QByteArray{};
         return dst;
     }