tst_QString/tst_QByteArray: add checks for null-ness

We want to preserve nullness where possible. Test that various ctors
do the right thing when presented with null input.

Pick-to: 6.3
Change-Id: Ia1a1d4fb3c919b4fed2d9b87827815a1b5072c54
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

tests/auto/corelib/text/qbytearray/tst_qbytearray.cpp

@@ -95,6 +95,7 @@ private slots:
     void number_double();
     void number_base_data();
     void number_base();
+    void nullness();
     void blockSizeCalculations();
 
     void resizeAfterFromRawData();
@@ -1362,6 +1363,38 @@ void tst_QByteArray::number_base()
     }
 }
 
+void tst_QByteArray::nullness()
+{
+    {
+        QByteArray ba;
+        QVERIFY(ba.isNull());
+    }
+    {
+        QByteArray ba = nullptr;
+        QVERIFY(ba.isNull());
+    }
+    {
+        const char *ptr = nullptr;
+        QByteArray ba = ptr;
+        QVERIFY(ba.isNull());
+    }
+    {
+        QByteArray ba(nullptr, 0);
+        QVERIFY(ba.isNull());
+    }
+    {
+        const char *ptr = nullptr;
+        QByteArray ba(ptr, 0);
+        QVERIFY(ba.isNull());
+    }
+    {
+        QByteArrayView bav;
+        QVERIFY(bav.isNull());
+        QByteArray ba = bav.toByteArray();
+        QVERIFY(ba.isNull());
+    }
+}
+
 static bool checkSize(qsizetype value, qsizetype min)
 {
     return value >= min && value <= std::numeric_limits<qsizetype>::max();

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

@@ -515,6 +515,7 @@ private slots:
     void macTypes();
     void isEmpty();
     void isNull();
+    void nullness();
     void acc_01();
     void length_data();
     void length();
@@ -1090,6 +1091,51 @@ void tst_QString::isNull()
 
 QT_WARNING_POP
 
+void tst_QString::nullness()
+{
+    {
+        QString s;
+        QVERIFY(s.isNull());
+    }
+    {
+        QString s = nullptr;
+        QVERIFY(s.isNull());
+    }
+    {
+        const char *ptr = nullptr;
+        QString s = ptr;
+        QVERIFY(s.isNull());
+    }
+#ifdef __cpp_char8_t
+    {
+        const char8_t *ptr = nullptr;
+        QString s = ptr;
+        QVERIFY(s.isNull());
+    }
+#endif
+    {
+        QString s(nullptr, 0);
+        QVERIFY(s.isNull());
+    }
+    {
+        const QChar *ptr = nullptr;
+        QString s(ptr, 0);
+        QVERIFY(s.isNull());
+    }
+    {
+        QLatin1String l1;
+        QVERIFY(l1.isNull());
+        QString s = l1;
+        QVERIFY(s.isNull());
+    }
+    {
+        QStringView sv;
+        QVERIFY(sv.isNull());
+        QString s = sv.toString();
+        QVERIFY(s.isNull());
+    }
+}
+
 void tst_QString::isEmpty()
 {
     QString a;