QString: fix append(const QStringRef &str)

Use QStringRef::isNull instead of QStringRef::string()
for validation. Non-NULL str.string() may yet leave us
with a useless str.unicode(), which is the actual problem here;
whereas !str.isNull() does really confirm that str.unicode()
is sensible.

Such test prevents situation like:

const QString a;
QString b;
b.append(a); // b.isNull() == true
b.append(QStringRef(&a)); // b.isNull() == false

Auto test updated: create QStringRef from QString directly, without
any condition.

Change-Id: I082cd58ef656d8a53e3c1223aca01feea82fffb9
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/tools/qstring.cpp

@@ -9375,7 +9375,7 @@ QString &QString::append(const QStringRef &str)
 {
     if (str.string() == this) {
         str.appendTo(this);
-    } else if (str.string()) {
+    } else if (!str.isNull()) {
         int oldSize = size();
         resize(oldSize + str.size());
         memcpy(data() + oldSize, str.unicode(), str.size() * sizeof(QChar));

tests/auto/corelib/tools/qstring/tst_qstring.cpp

@@ -131,7 +131,7 @@ template <>
 class Arg<QStringRef> : ArgBase
 {
     QStringRef ref() const
-    { return this->pinned.isNull() ? QStringRef() : this->pinned.midRef(0) ; }
+    { return QStringRef(&pinned); }
 public:
     explicit Arg(const char *str) : ArgBase(str) {}