fix: guard null restore target in FocusScope restoreFocus (#10371)
getFirstInScope() returns the result of TreeWalker.nextNode(), which is null
when the scope contains no focusable element, for example when its focusable
content has been removed or hidden. The restoreFocus fallback passed that
value straight into restoreFocusToElement(), throwing a TypeError on a null
dereference. Correct the misleading return type and skip scopes with nothing
to restore to, continuing up the focus scope tree instead.
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>