Incorrect command-line parsing if argc=0

I'm experiencing problems with QtWebEngine not detecting QTWEBENGINE_CHROMIUM_FLAGS parameters if QGuiApplication have been created with argc=0. Zero command-line arguments might sound kind of weird but ActiveQt does exactly that (see QTBUG-110158), which is what is triggering this problem.

Steps to reproduce:

• Open WebEngineQuick "minimal" sample
• Modify main function to the following:

int main(int argc, char *argv[])
{
    // Run web engine in-proc (to avoid QtWebEngineProcess[d].exe)
    qputenv("QTWEBENGINE_CHROMIUM_FLAGS", "--single-process");

    QCoreApplication::setOrganizationName("QtExamples");
    QCoreApplication::setAttribute(Qt::AA_ShareOpenGLContexts);
    QtWebEngineQuick::initialize();

    argc = 0; // pretend zero command-line arguments (to mimic ActiveQt)
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

    return app.exec();
}

• Observe that two QtWebEngineProcess[d].exe processes are started in addiction to webengine-minimal-qml.exe, despite WebEngine being instructed to run in-process.

Suggested fix

Improve WebEngineContext::initCommandLine to better deal with situations where QCoreApplication::arguments() returns an empty list.