wayland/screencopy: avoid crash when capturing null screen

[imalbi]

Overview

Avoid null pointer dereference crashes when attempting to capture screencopy on a null screen (e.g. Quickshell.screens[0] when no screens are connected or during DRM hotplug/reconnect).

Details

• Check that screen->screen != nullptr in ScreencopyManager::createContext().
• Check that screen != nullptr before calling screen->handle() in IccOutputSourceManager::captureOutput() and WlrScreencopyManager::captureOutput().
• Returning nullptr lets ScreencopyView warn and idle safely until a valid screen is connected and re-evaluated by QML.

Requirements

• I have read and agree with the contributing guidelines https://github.com/quickshell-mirror/quickshell/blob/master/CONTRIBUTING.md