QXmlString: add a few missing operations

... namely isNull() and op==/!=().

These will be needed for porting comparesEqual() from public getters
to direct data member access, in order to make them earn their
noexcept tagging.

Change-Id: Id8677d39dc581d03adfd3d0d420907ff60c0423a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit e0ae91bd5880d1f8ab36d42cff428bb8246f73c2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/serialization/qxmlstream.h

@@ -33,6 +33,14 @@ public:
 
     operator QStringView() const noexcept { return QStringView(m_string.data(), m_string.size); }
     qsizetype size() const noexcept { return m_string.size; }
+    bool isNull() const noexcept { return m_string.isNull(); }
+
+private:
+    friend bool comparesEqual(const QXmlString &lhs, const QXmlString &rhs) noexcept
+    {
+        return QStringView(lhs) == QStringView(rhs);
+    }
+    Q_DECLARE_EQUALITY_COMPARABLE(QXmlString)
 };
 
 }