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/gui/DatabaseOpenWidget.cpp

@@ -122,7 +122,7 @@ void DatabaseOpenWidget::hideEvent(QHideEvent* event)
 
 #ifdef WITH_XC_YUBIKEY
     // Don't listen to any Yubikey events if we are hidden
-    disconnect(YubiKey::instance(), 0, this, 0);
+    disconnect(YubiKey::instance(), nullptr, this, nullptr);
     m_yubiKeyBeingPolled = false;
 #endif
 }