QLatin1String: add missing isNull(), isEmpty()

All Qt string containers have them, so tst_qstringapisymmetry pointed out
their absence.

[ChangeLog][QtCore][QLatin1String] Added isEmpty(), isNull().

Change-Id: I5ed27ed036c29a7d0f041910fce6c27d9270dd26
Reviewed-by: Olivier Goffart (Woboq GmbH) <ogoffart@woboq.com>

src/corelib/tools/qstring.cpp

@@ -8366,6 +8366,24 @@ QString &QString::setRawData(const QChar *unicode, int size)
     Returns the size of the Latin-1 string stored in this object.
 */
 
+/*! \fn bool QLatin1String::isNull() const
+    \since 5.10
+
+    Returns whether the Latin-1 string stored in this object is null
+    (\c{data() == nullptr}) or not.
+
+    \sa isEmpty(), data()
+*/
+
+/*! \fn bool QLatin1String::isEmpty() const
+    \since 5.10
+
+    Returns whether the Latin-1 string stored in this object is empty
+    (\c{size() == 0}) or not.
+
+    \sa isNull(), size()
+*/
+
 /*! \fn QLatin1Char QLatin1String::at(int pos) const
     \since 5.8
 

src/corelib/tools/qstring.h

@@ -97,6 +97,9 @@ public:
     Q_DECL_CONSTEXPR int size() const Q_DECL_NOTHROW { return m_size; }
     Q_DECL_CONSTEXPR const char *data() const Q_DECL_NOTHROW { return m_data; }
 
+    Q_DECL_CONSTEXPR bool isNull() const Q_DECL_NOTHROW { return !data(); }
+    Q_DECL_CONSTEXPR bool isEmpty() const Q_DECL_NOTHROW { return !size(); }
+
     Q_DECL_CONSTEXPR QLatin1Char at(int i) const { return QLatin1Char(m_data[i]); }
     Q_DECL_CONSTEXPR QLatin1Char operator[](int i) const { return at(i); }