clutter/stage: Issue a warning if entry is NULL rather than crashing

This should give us better insight into what's causing the condition rather than incomplete crash reports from optimized release builds.

This is the top crasher of gnome-shell 45 in Ubuntu 23.10:
https://launchpad.net/bugs/2035096
https://errors.ubuntu.com/problem/430fb8d5733006abef7cc611f1b1ed57b0c6b0a5