cocoawindowcontext: Skip unavailable system buttons (fixes crash)

[Zoinen]

Summary

• Skip unavailable macOS standard window buttons when constructing the observer list.
• Prevent an Objective-C exception when a tool or modal window does not provide all three traffic-light buttons.

-[NSMutableArray addObject:] does not accept nil, while standardWindowButton: may return nil for buttons omitted by a particular window style.

Testing

Configured and built QWindowKit on macOS with:

• QWINDOWKIT_BUILD_QUICK=ON
• QWINDOWKIT_BUILD_WIDGETS=OFF
• QWINDOWKIT_BUILD_EXAMPLES=OFF

Both QWKCore and QWKQuick built successfully.