QString/QByteArray::op>>: don't assume clear() makes the container isNull()

This is something we may want to change at some point, so be explicit
and assign a null container instead of relying on clear().

Add a comment that was present in the QString, but missing in the
QByteArray implementation.

Task-number: QTBUG-31283
Task-number: QTBUG-60745
Change-Id: I10d82b8a0c67fff314af526c7b7e0f247c3405fd
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
(cherry picked from commit 038c1c5978c30675c5f953035a193bc76ca6ca21)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/text/qbytearray.cpp

@@ -3229,8 +3229,10 @@ QDataStream &operator>>(QDataStream &in, QByteArray &ba)
     ba.clear();
     quint32 len;
     in >> len;
-    if (len == 0xffffffff)
+    if (len == 0xffffffff) { // null byte-array
+        ba = QByteArray();
         return in;
+    }
 
     const quint32 Step = 1024 * 1024;
     quint32 allocated = 0;

src/corelib/text/qstring.cpp

@@ -10432,7 +10432,7 @@ QDataStream &operator>>(QDataStream &in, QString &str)
         quint32 bytes = 0;
         in >> bytes;                                  // read size of string
         if (bytes == 0xffffffff) {                    // null string
-            str.clear();
+            str = QString();
         } else if (bytes > 0) {                       // not empty
             if (bytes & 0x1) {
                 str.clear();