layout: Check again for primary monitor after startup animation idle

layout: Check again for primary monitor after startup animation idle

On some hardware combinations the display can be known, then unknown again. Meaining that
when the update monitors function is called it will have a value, then be called again
setting this.primaryMonitor to null. If the timing is just right gnome shell will
loadBackground, then by the time the animation is ready the monitor will be gone,
thus methods will be called on a null value. This adds more checks for a valid
primary monitor, and wont play the animation until the system is idle AND has a valid
priamry monitor.

Fixes: #5003
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2144>