Improve readability and type-safety

Use nullptr instead of 0 or NULL to initialize a null pointer. In some
cases, readability was enhanced by replacing 0 with more meaningful
values according to the type of the pointer being initialized.

src/browser/HostInstaller.cpp

@@ -111,7 +111,7 @@ void HostInstaller::installBrowser(SupportedBrowsers browser,
         // Always create the script file
         QJsonObject script = constructFile(browser, proxy, location);
         if (!saveFile(browser, script)) {
-            QMessageBox::critical(0,
+            QMessageBox::critical(nullptr,
                                   tr("KeePassXC: Cannot save file!"),
                                   tr("Cannot save the native messaging script file."),
                                   QMessageBox::Ok);