QLatin1String: add default ctor

A pending change in uic depends on this.

[ChangeLog][QtCore][QLatin1String] Added default constructor.

Change-Id: Ie6f5dfc7b38683a488b0ff7f31404800ef5ee188
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qstring.cpp

@@ -8225,6 +8225,12 @@ QString &QString::setRawData(const QChar *unicode, int size)
     \sa QString, QLatin1Char, {QStringLiteral()}{QStringLiteral}
 */
 
+/*! \fn QLatin1String::QLatin1String()
+    \since 5.6
+
+    Constructs a QLatin1String object that stores a nullptr.
+*/
+
 /*! \fn QLatin1String::QLatin1String(const char *str)
 
     Constructs a QLatin1String object that stores \a str.

src/corelib/tools/qstring.h

@@ -83,6 +83,7 @@ template <typename T> class QVector;
 class QLatin1String
 {
 public:
+    Q_DECL_CONSTEXPR inline QLatin1String() Q_DECL_NOTHROW : m_size(0), m_data(Q_NULLPTR) {}
     Q_DECL_CONSTEXPR inline explicit QLatin1String(const char *s) : m_size(s ? int(strlen(s)) : 0), m_data(s) {}
     Q_DECL_CONSTEXPR inline explicit QLatin1String(const char *s, int sz) : m_size(sz), m_data(s) {}
     inline explicit QLatin1String(const QByteArray &s) : m_size(s.size()), m_data(s.constData()) {}

tests/auto/corelib/tools/qlatin1string/tst_qlatin1string.cpp

@@ -46,6 +46,16 @@ private Q_SLOTS:
 
 void tst_QLatin1String::nullString()
 {
+    // default ctor
+    {
+        QLatin1String l1;
+        QCOMPARE(static_cast<const void*>(l1.data()), static_cast<const void*>(Q_NULLPTR));
+        QCOMPARE(l1.size(), 0);
+
+        QString s = l1;
+        QVERIFY(s.isNull());
+    }
+
     // from nullptr
     {
         const char *null = Q_NULLPTR;