QWinRegistryKey: fix assert when querying default value

I wrongly assumed we can't query a value with an empty name ""
during the previous refactor commit, however, in Windows registry,
an empty name for a value means the default value of a key, we can
read and write it through the "Default" name.
Remove the wrong assert to fix the crash when we are trying to query
a default value of a key.

Add a new test case to test this kind of scenarios.

Amends commit 40523b6

Fixes: QTBUG-107794
Change-Id: Idacbcb86df4435a8c1ca1c19121599390ae8f3d3
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/corelib/kernel/qwinregistry.cpp

@@ -38,11 +38,12 @@ void QWinRegistryKey::close()
 
 QVariant QWinRegistryKey::value(QStringView subKey) const
 {
-    Q_ASSERT(!subKey.isEmpty());
-
     if (!isValid())
         return {};
 
+    if (subKey.isEmpty())
+        subKey = u"Default";
+
     auto subKeyC = reinterpret_cast<const wchar_t *>(subKey.utf16());
 
     // Get the size and type of the value.

tests/auto/corelib/kernel/qwinregistrykey/tst_qwinregistrykey.cpp

@@ -20,6 +20,7 @@ static const QPair<QStringView, quint32> TEST_DWORD = qMakePair(u"dword", 123);
 static const QPair<QStringView, quint64> TEST_QWORD = qMakePair(u"qword", 456);
 static const QPair<QStringView, QByteArray> TEST_BINARY = qMakePair(u"binary", "binary\0"_ba);
 static const QPair<QStringView, QVariant> TEST_NOT_EXIST = qMakePair(u"not_exist", QVariant());
+static const QPair<QStringView, QVariant> TEST_DEFAULT = qMakePair(u"Default", u"default"_s);
 
 [[nodiscard]] static inline bool write(const HKEY key, const QStringView name, const QVariant &value)
 {
@@ -124,6 +125,8 @@ void tst_qwinregistrykey::initTestCase()
         return;
     if (!write(key, TEST_BINARY.first, TEST_BINARY.second))
         return;
+    if (!write(key, TEST_DEFAULT.first, TEST_DEFAULT.second))
+        return;
     m_available = true;
 }
 
@@ -141,9 +144,10 @@ void tst_qwinregistrykey::cleanupTestCase()
     RegDeleteValueW(key, C_STR(TEST_DWORD.first));
     RegDeleteValueW(key, C_STR(TEST_QWORD.first));
     RegDeleteValueW(key, C_STR(TEST_BINARY.first));
+    RegDeleteValueW(key, C_STR(TEST_DEFAULT.first));
     #undef C_STR
-    RegCloseKey(key);
     RegDeleteKeyW(HKEY_CURRENT_USER, TEST_KEY);
+    RegCloseKey(key);
 }
 
 void tst_qwinregistrykey::qwinregistrykey()
@@ -206,6 +210,18 @@ void tst_qwinregistrykey::qwinregistrykey()
         QCOMPARE(value.value_or(QVariant()), QVariant());
     }
 
+    {
+        const auto value = registry.value<QString>(TEST_DEFAULT.first);
+        QVERIFY(value.has_value());
+        QCOMPARE(value.value_or(QString()), TEST_DEFAULT.second);
+    }
+
+    {
+        const auto value = registry.value<QString>(L"");
+        QVERIFY(value.has_value());
+        QCOMPARE(value.value_or(QString()), TEST_DEFAULT.second);
+    }
+
     {
         const QString value = registry.stringValue(TEST_STRING.first);
         QVERIFY(!value.isEmpty());