Fix null error related to the behavior of ResizeObserver on Firefox & Safari

[InfiniteRain]

This PR fixes an issue which exists on Firefox and Safari which throws a null error whenever the foliate-view element is destroyed. I assume this happens because these browsers don't automatically clean up the ResizeObserver callbacks once their underlying elements have been destroyed. In any case, adding a simple null check in paginator.js's expand() method fixes the issue.

[johnfactotum]

foliate-view has a close() method that calls destroy() on the paginator which then unobserves the document:

foliate-js/view.js

Lines 295 to 297 in bdae473

	close() {
	this.renderer?.destroy()
	this.renderer?.remove()